Methods and apparatuses for encoding an HDR images, and methods and apparatuses for use of such encoded images

ABSTRACT

Encoding a high dynamic range image (M_HDR), comprising: 1. converting the high dynamic range image to an image of lower luminance dynamic range (LDR_o) by applying the acts of: a) scaling the high dynamic range image to a predetermined scale of the luma axis such as [0,1], b) a sensitivity tone mapping which changes the brightnesses of pixel colors falling within at least a subrange comprising the darker colors in the high dynamic range image, c) a gamma function, and d) applying an arbitrary monotonically increasing function mapping the lumas resulting from performing the acts b and c to output lumas of the lower dynamic range image (LDR_o); and 2. outputting in an image signal (S_im) a codification of the pixel colors of the lower luminance dynamic range image (LDR_o), and 3. outputting in the image signal (S_im) values encoding the functional behavior of the above color conversions as metadata, or values for the inverse functions, which metadata allows to reconstruct a high dynamic range image (Rec_HDR) from the lower luminance dynamic range image (LDR_o).

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2015/055831, filed on Mar.19, 2015, which claims the benefit of European Patent Application No.14170157.3, filed on May 28, 2014. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to encoding of one (i.e. a still) but preferablymore (i.e. video) High Dynamic Range image(s), and correspondingtechnical systems and methods to convey the necessary coded imageinformation to a receiving side, and decoders to decode the codedimages, and ultimately make them available for display.

A HDR image is an image which encodes the textures of a HDR scene (whichmay typically contain both very bright and dark regions), in such amanner, with sufficient information for high quality encoding of thecolor textures of the various captured objects in the scene, that avisually good quality rendering of the HDR scene can be done on a HDRdisplay with high peak brightness like e.g. 5000 nit. In our HDRencoding framework developed over the previous years, we also want atthe same time to encode various encompassed dynamic range subviews onthat scene, which can serve as good driving images for various displaysof successively reduced dynamic range, at least an LDR image looksuitable for driving e.g. a 100 nit peak brightness display.

Furthermore an HDR encoding is preferably technically designed so thatit not only matches relatively well with existing video codingtechnologies, but even can fit into the current image or video encodingframeworks of existing technology, like e.g. blu-ray disk storage (andits limitations like amount of storage memory, but also alreadysubstantially all other standardized aspects), or HDMI cableconnections, or other image transmission or storage systems, etc. Inparticular, HDR encoding systems which use two encoded images per timeinstant (e.g. an LDR image, and an image having local brighteningintensities for boosting each pixel to an HDR image pixel) are seen byespecially apparatus manufacturers as needlessly complex.

HDR video (or even still image) encoding has only recently beententatively researched and has been a daunting task up to now, and thetypical belief is that one either needs to go towards significantly morebits, for encoding the brightnesses above the LDR range of scene objects(e.g. encodings which encode scene luminances directly), or one needssome two-layer approach, wherein e.g. in addition to an objectreflectance image there is a illumination boost image, or similardecomposition strategies. Alternative approaches, like e.g. the HDRcoding system of Dolby, typically always use such a two-layer HDRencoding technology (see U.S. Pat. No. 8,248,486B1). Wo2010/105036 isanother example of such a 2-image-per-time-instant encoding system fromDolby. Some simple fixed tone mapping TM (e.g. emulating the behavior ofclassical analog celluloid photography) can be used to map a LDR imageto a HDR grading corresponding therewith, or vice versa. This may be anynot very correctly mapping function (because the grader may have usedcomplicated optimization decisions in say his LDR grading), so there maybe a considerable difference between the prediction of the original sayHDR image, and the original HDR image itself, but that is no problembecause the system can send the differences as a second correction image(residual bit stream 742).

Another example of such a two-image encoding system is the technicolorHDR coding system (document JCTVC-P0159r1, of the 16^(th) meeting of theJoint Cooperative Team on video coding of ITU-T SG 16 WP3, San José of9-17 Jan. 2014) which encodes as a first image a low resolution signalbeing the local illumination as it will typically be generated in amodulated 2D LED backlight, and the second image is a texture image oftypically lower dynamic range modulations upon that low frequencysignal, which will typically be generated upon display by appropriateLCD valve driving signals.

Philips has recently invented a much simpler single image approach(still largely unpublished, but some aspects can be found inWO2012/147022 and WO2012/153224), which is a new direction in image andvideo coding, and not only a priori not easy to imagine, but also whenactually doing it leading to many new technical issues to be solved,which however works nicely in practice.

With “high dynamic range” (HDR) we mean that either the image(s) ascaptured from the capturing side have a high luminance contrast ratiocompared to legacy LDR encoding (i.e. object luminance contrast ratiosof 10.000:1 or more may be handled by the coding, and all components ofthe image handling chain up to rendering; and captured object luminancesmay be above 1000 nit, or more specifically, may typically bereproduced/rendered above 1000 nit to, given the reproductionenvironment, generate some desired appearance of say a lit lamp or sunnyexterior). And typically the rendering of such image(s) is HDR (i.e. theimages must be suitable in that they contain information sufficient forhigh quality HDR rendering, and preferably in a technically easy to usemanner), meaning the image(s) are rendered or intended to be rendered ondisplays with peak brightness of at least 1000 nit (not implying theycan't and needn't alternatively be rendered on LDR displays of e.g. 100nit peak brightness, typically after suitable color mapping).

BACKGROUND OF THE INVENTION

Recently a number of HDR encoding technologies have been proposed, likee.g. the dual layer method of Dolby (WO2005/1040035). However, theindustry is currently still looking for a pragmatic HDR video (/image)encoding technology with fits with (a balance of) all requirements, suchas the very important factors like amount of data but also computationalcomplexity (price of ICs), ease of introduction, versatility for theartists to create whatever they like, etc. In particular, a dual layerapproach is seen as needlessly complex. One would ideally like to beable to design a coding technology which fits with legacy encoding, suchas e.g. DCT-based MPEG HEVC encoding. A problem is that this is somewhatcounter-intuitive: how can one encode a HDR image, which should bydefinition be something different from an LDR image, typically having alarger amount of interesting brightness/luminance ranges, in atechnology optimized for containing LDR images? These legacy LDR imagehandling/coding systems were designed and optimized to work with typicalLDR imaging scenarios, which are normally well-lit with e.g. a 4:1 instudio illumination ratio (or e.g. 10:1), giving for most of the objects(which can vary in reflectance between say 85% for white and 5% forblack) in the view a total contrast ratio of about 68:1 (resp. 170:1).If one looks at relative rendering (i.e. mapping the image white to theavailable display white) of the object luminances starting from a peakwhite, a typical early LCD monitor without local dimming would have hadsomething like 100 nit white and 1 nit black which would match with theimage contrast ratio, and typically one thought that on average CRTsystems which might have been watched also during the day would havesomething like a 40:1 capability. Having a standard legacy luminancecode allocation gamma function of 2.2 in these systems seemedsatisfactorily for most scenarios of even higher scene contrast.Although some in those days regarded as acceptable errors were made,such errors of rendering of badly encoded high luminance scene regions(e.g. hard clipping of bright exteriors behind a window) were alsoacceptable because LDR displays couldn't render those object luminancesphysically accurate anyway.

However there are scenarios for which there is now a desire to improvethe rendering, like e.g. an indoors scene in which one cansimultaneously see the sunny outdoors, in which case there may be anillumination ratio of 100:1 or even more. With linear relative rendering(i.e. focusing on the brightest encoded regions firstmost, orequivalently the middle grey of the brightest scene regions, and mappingimage white to display white), the indoors white would map topsychovisual black to the viewer! So in LDR those sunny regions willtypically show up as (soft)clipped (typically already in the encodedimage having difficult to discriminate codes around the maximum 255 forthose pixels). However, on a HDR display we would like to show them bothbright and colorful. That would give a much more naturalistic andspectacular rendering of such scenes (as if you're really on holiday inItaly), but even scenes in which the higher brightness content is onlycomposed of some specular reflections already show a major visualquality improvement. If not already artefacts like clipping orquantization errors look annoying on e.g. a 5000 or 10000 nit display,at least we want to be able to drive such HDR displays with the rightkind of image, so that the rendered images will be as beautiful as theHDR display allows.

Classical thinking was however that to encode additional over-brightnessranges, one would need to have (much) more bits, which are the higherbits which encode the object luminances above an LDR range. That couldhappen either by natively encoding in single larger code words (such asOpenEXR with 16 bits of which a sign bit, 5 bits exponent, and 10 bitsmantissa, or Ward's LogLuv encoding, which mathematically rigorouslytries to capture the entire world of possible object luminances withhigh precision), or by using a first layer with standard LDR range codes(e.g. a classical JPEG approximation of the HDR image), and a secondlayer to improve such pixel luminances to higher brightness (e.g. aboost image to boost each pixel if needed to a higher luminance, i.e. amultiplication of two such 8 bit images being equivalent to a singlelinear 16 bit code).

A major practical problem to be solved when designing a practical HDRcoding technology, in addition to the fact that of course it must beable to handle a huge range of different HDR images, is that hardwaremanufacturers desire lower amounts of bits per code word (channel, i.e.the luma, and two chromatic channels) however, and although our belowproposed technology can also work with larger bit words, we come with asolution that works nicely under a limitation of 10 bits for at least aluminance (or more precisely a luma) channel (note that although weelucidate the embodiments with a luminances channel, our concepts maymutatis mutandis be embodied as working on (linear or non-linear) RGBcolor representations, etc.). Furthermore, we developed a frameworkwhich can do in a dual philosophy both the color pixels encoding (of theHDR look via an LDR image) and the color appearance conversion forseveral rendering scenarios (i.e. the needed optimal looks for renderinga scene on several displays with different peak brightness, e.g PB=800nit) in a functional manner, which means only functions need to beco-encoded when encoding the look of at least one further grading, andspecifically an HDR look in addition to an LDR look, instead of for eachpicture at least a second picture. We have currently two categories ofHDR encoding systems, since the market would like such versatility in anencoding system, given the various players and their different needs. Inthe mode-i (or HDR-look encoded as a sole defining image, e.g. on a BDdisk, or an stream of AVC or HEVC images over a network connection)system we use a HDR-look image as the sole pixel image, which is used toencode the object color textures and shapes (see in WO2015007505 ofapplicant how such a sole HDR image can be sent to a receiver to definethe pixel colors of at least the HDR look, and how with appropriatere-grading functions the receiver can calculate by processing the colorsin that image other look images). By this we mean that we take theoriginal HDR master grading image, i.e. an image optimally color gradedto look best on a reference HDR display like e.g. typically a 5000 nitpeak brightness display, and only minimally transform this: basicallyonly apply a code allocation function or Opto-electronic transferfunction OETF (note that although this OETF defines how scene luminancesas captured e.g. by a camera are transferred to luma codes, televisionengineers instead like to specify the inverse function being theelectro-optical transfer function EOTF to go from luma codes toreference display rendered luminances) by using the OETF optimallyallocates the available e.g. 10 bit of codes for the luma Y′ channelover all brightness values one needs to be able to make on a reference[0-5000] nit display. Other desired gradings for displays of differentpeak brightness can then be made by transforming this HDR-look image. Inour framework we allow for this display look tunability by typicallymaking only one second grading which is on an other extreme end of therange of possible displays to be served, namely a look which is optimalor reasonable according to the content creator/color grader for a 100nit peak brightness display (which is typically the reference displayfor the category of LDR displays). Note that this is a co-encoding of afurther look rather than a mere creation-side recoloring step. Thisrequired color transformation is determined by applying mappingfunctions such as gamma functions realizing a global brightnessreadjustment (e.g. brightening the darker colors in the image),arbitrary S-shaped or inverse S-shaped curves to adjust local contrast,color saturation processing functions to adjust e.g. the saturation tothe corresponding brightness of some objects or regions in the imageetc. We can liberally co-encode those functions (whichever functions weneed as long as they belong to a limited set of basis functions whichthe receiver can in a standardized manner understand) as metadataassociated with the pixellized HDR-look image, in which case weparametrically DEFINE the second LDR-look grading image from theHDR-look image (i.e. we need not encode that LDR-look image as a pixelimage anymore). Note carefully the difference with two layer encodingsystems: in our system the color transformation functions are all thereis encoded about the second look to be able to re-grade the second lookat the receiver, so rather than the rough approximate functions of2-image technologies, our functions contain the full smart knowledge ofhow the illuminations of the various objects should behave in variousrendering look according to the content creator! Given this knowledge ofhow the creating artists wants the look to transform from the first lookfor displays with a first level of color rendering capabilities to asecond look for displays with a second level of color renderingcapabilities (in particular the display peak brightness), a display withintermediate capabilities (e.g. 1200 nit peak brightness) can thenautomatically come to a more optimal driving image for its renderingsituation by using the knowledge in the two gradings and interpolating(e.g. the display may do an asymmetric mixing of the two pixellizedimages of the HDR-look and the derived LDR-look image from the HDR-lookimage and the functional transformations, in which the multiplicativemixing percentages are determined by how close to the HDR or LDR displaythe actual display is on a psychovisual non-linear scale), which will bebetter than driving the display with either the original HDR-look imageor the LDR-look image.

This is a powerful yet simple definition of not solely a single (HDR)image look on a scene (e.g. a 5000 nit rendering), but a full frameworkfor deriving reasonable renderings of the scene for various possibledisplays in the field like at a consumer's home (and even potentiallyadaptation to viewing environment e.g. by applying a post-gamma modelingthe changed contrast sensitivity of human vision under various surroundilluminances). It is mainly useful e.g. for applications/scenarios inwhich a creator has made a nice HDR version of their content, and wantsto have firstmost this HDR look in the actual encoding sent to receivers(e.g. on a HDR BD disk, or by ordering a HDR movie online over theinternet, or a HDR television broadcast, etc.). It is not necessary thata customer who purchases this content version actually has a HDRdisplay, since he can purchase it for later when he does have a HDRdisplay and can now use the HDR-2-LDR conversion, but it would be thepreferred option when the customer wants content for his HDR display.

Whereas the above HDR-look manner of encoding HDR scenes (as explainedmode i being that at least HDR look images encoded as a pixel image, butin fact also further looks on that same scene are encoded but thenparametrically with color transformation functions, such as e.g. aclipping embodiment, in which the LDR-look isolates a subrange of theHDR image and clips the rest) already poses significant technicalchallenges for coming to a pragmatic new technical system for futureimage but mostly also video encoding (taking into account such factorsas simplicity of IC design for the hardware manufacturers, yet allowingcontent makers to create whatever beautiful HDR content like scifimovies, spectacular television shows, or nature documentaries, etc. theywant to make, with many creative HDR effects such as lamps which seemreally lit), the market desired yet another layer of complexity, whichwe will teach in this patent description.

Namely, for some (which we will call mode-ii) applications one may wantto have an LDR-look image as the sole pixellized image encoding thescene objects, which is e.g. written as sole image on a blu-ray disk.Although the content creator also cares much about the quality of theHDR look, he very much focuses on the LDR look being similar as it wouldbe with legacy technologies. There will then typically be functionparameters co-encoded in associatable metadata to derive a HDR lookimage by upgrading the LDR-look image which was communicated in theimage signal S_im. There may be various reasons for choosing thismode-ii variant (or LDR-look), which may e.g. be for legacy systemswhich are unable to do any processing (e.g. if one prefers to encode thesole image in a particular embodiment which encodes the colors as Y′uvcolors rather than a YCrCb encoding, one could still encode this in alegacy HEVC framework by pretending the Y′uv image is a strangelycolored YCrCb image and further using legacy DCT-based encoding schemes,like standardized in one of the members of the MPEG codec family), butalso for applications which need a LDR look (e.g. viewing a movie on alow brightness portable display) and may not want to do too muchprocessing. Or perhaps the creator doesn't want to invest too much timein creating a perfect HDR look (but e.g. only a quickly makes one bydoing minor fine tuning of an LDR-2-HDR auto conversion which e.g.isolates bright regions and non-linearly boosts them, e.g. for an oldLaurel and Hardy movie remastering), and considers his LDR-look the mostimportant master grading of the LDR and HDR looks, which should bedirectly encoded without needing any color transformation, withpotential color errors. E.g. a television broadcaster may choose thisoption, especially for real-life broadcasts (e.g. the news may not needto be in the most spectacular HDR).

This LDR-look (mode ii) encoding however has additional complexity dueto the mathematical nature of the problem and coding mathematics on theone hand versus liberal artistic grading desires on the other, whichmakes it a daunting task to come up with a good technical framework. Tobe more precise, on the one hand we need functions which first gradedown from a desired master HDR image, and at the receiver with thesereceived functions (or the inverse functions of the downgradingactually) the receiver can upgrade to at least a close approximation ofthe original HDR image again, i.e. in the metadata function parameterdata there will be parameters for functions (derived by the encoder fromthe functions which the grader used in the downgrading from the masterHDR) which can map the sole LDR image to a sufficiently close HDRprediction Rec_HDR. But on the other hand, the LDR image should whendirectly rendered on a +−100 nit display, i.e. without further colortransformation, look sufficiently good according to the color gradertoo. So there will be a balance between selection of the functions, andhow they will influence the LDR and Rec_HDR looks, and that also takinginto account other issues, like that IC or apparatus manufacturers wouldlike to see a limited set of standard functions which are useful for there-grading of looks, and content creators like those functions toquickly specify whatever looks they desire, since grading time isexpensive and the timing of movie releases may be critical. In the belowdescription we will describe a practical system for handling this modeii variant of HDR scene encoding.

SUMMARY OF THE INVENTION

We need to have an improved encoding of HDR images, and in particular,we started with the philosophy that especially at the current momentwhen there are still many legacy LDR systems out there in the field, oneneeds some levels of compatibility. This means on the one hand that wewould like to keep using existing (de)coder ICs which implementfunctionality like (I)DCT [=first level compatibility with imagecommunication technologies]. But in addition there needs to be secondlevel compatibility with displays which because of their low peakbrightness need LDR images (i.e. an LDR look, not an HDR look with e.g.too dark colors in the darker parts of the image, but rather with darkercolors which have been brightened up for better visibility on LDRdisplays) because they can only render LDR images (i.e. the correct LDRlook under such a display dynamic range capability). This is because inaddition to the presently deployed legacy TVs, in the further futurethere will be a spectrum of displays ranging from low brightnesscapability small portable displays like laptop or pad-computers or evenmobile phones on which a consumer also desires to see some rendering ofa HDR movie, up to the most advanced HDR displays, which in the futuremay have a peak brightness of e.g. 10000 nit, and all displays inbetween or around those. Then although the display may still be legacyand simple, it could be served by a high complexity new decoding andcolor mapping IC in e.g. a future settopbox or computer supplying theHDR content via e.g. a HDMI or other connection, that settopbox offeringany combination of the options we invented and described. Note that alegacy LDR image would need to be some optimization between intra-objectand inter-object contrast. We would like to see the interior textures ofobjects clearly, yet still also want to have in the LDR image animpression of the maybe huge HDR contrasty look of the original scene.I.e. the difference between a region of high and low brightness may notbe renderable perfectly with the LDR image, yet there should still be aremnant of this, making the illumination changes in the scene still asoptimal as possible conveyable in LDR by the human grader.

We have converted these requirements into an approach in which one wouldin the ideal scenario need (at least) two gradings for the same movie orpictures from the content provider, which we will simply call an LDRimage (to be used for LDR display scenarios, e.g. with displays withpeak brightness around 100 nit) and an HDR image (for the brighterdisplays, e.g. a reference display of 5000 nit peak brightness).

So for several practical example scenarios we have as starting point forthe novel HDR encoding embodiments as input a master HDR graded image(let's say it's graded at will according to whatever the creator's tastewas with whatever color processing software, and e.g. encoded in astarting color encoding like OpenEXR, and it may even be an upgrading toa HDR look of an image which was originally captured as LDR, e.g. byadding computer graphics effects). We then need to encode this masterHDR (M_HDR) in a way which is practically usable for current video orimage coding technologies (i.e. only minorly modified from the normalway to use such coding technologies which may involve a redefinition ofthe code meanings i.e. the respective luminances encoded by various lumacodes, but not that e.g. all busses need to be changed to 12 bit, i.eour methods should work with 12 bit hardware, but also if only 10 bitper component is available, or if one accepts some lower quality even on8 bit systems), for e.g. a new BD-disk player, or television ICreceiving internet streamed video, or any receiver connected to whateverimage source largely compliant to a variant of current image/videoencoding technologies.

We have come to the important understanding that a HDR image can beencoded as a LDR look image (i.e. an image which with little or nocolorimetric processing—maybe a conversion to another color space, butnot any or much tone mapping to convert the brightnesses of the imageobjects to be more suitable for a display with another luminance dynamicrange—can be directly used for good quality display on an LDR display)if only one adds the parameters of the color mapping functions which canconvert that LDR look into an HDR look image (our mode ii). The readershould contemplate that this is not a trivial thing to do, eventheoretically, certainly not a priori, but even after having set thetechnical task (because it would without the correct further developmentseem somewhat contradictory to encode one look via another look which issupposed to be different). In particular, since many of our embodimentsstart from an existing M_HDR, the functions can map the encoded LDR lookpixellized image into a close reconstruction Rec_HDR of the M_HDR. Butof course this can generically not be just done in any particular ad hocmanner, i.e. a specific technical encoding chain is needed.

-   -   Our invention can be realized e.g. in at least the following        ways: A method of encoding a high dynamic range image (M_HDR),        comprising the steps of:    -   converting the high dynamic range image to an image of lower        luminance dynamic range (LDR_o) by applying: a) normalization of        the high dynamic range image to a scale of the luma axis being        [0,1] yielding a normalized high dynamic range image with        normalized colors having normalized luminances (Yn_HDR), b)        calculating a gamma function on the normalized luminances        yielding gamma-converted luminances (xg), c) applying a first        tone mapping which boosts those gamma-converted luminances of        pixels of the normalized high dynamic range image which lie        below 0.1 with a predetermined amount lying between 1.5 and 5.0,        yielding lumas (v), and d) applying an arbitrary monotonically        increasing tone mapping function mapping the lumas resulting        from performing the steps b and c to output lumas (Yn_LDR) of        the lower dynamic range image (LDR_o); and    -   outputting in an image signal (S_im) a codification of the pixel        colors of the lower luminance dynamic range image (LDR_o), and    -   outputting in the image signal (S_im) values encoding the        function shapes of the above color conversions b to d as        metadata, or values for their inverse functions, which metadata        allows a receiver to reconstruct a reconstructed high dynamic        range image (Rec_HDR) from the lower luminance dynamic range        image (LDR_o).

This special combination of luma conversion functions has proven to be avery good way to handle encoding HDR images in the mode ii mindset, i.e.in particular in the dual way of encoding HDR images as LDR look imagesderived by these conversion functions from a master HDR, which LDRimages serve the dual purpose of faithfully encoding the HDR look aswell as being well-usable LDR look images.

Note that any codification of the pixel colors, i.e. representing thecolors of the pixels in some color space coding system can be used, andsome will be more pragmatic than others. E.g. the LDR_o could beoutputted as an (R′G′B′) image [wherein the dashes indicate somenon-linear mapping of linear RGB components]. We elucidate with anexample being able to encode the LDR image to be communicated to areceiver in a Yu′v′ manner, and then also the processing like the tonemapping may be performed in an Yxy representation with xy chromaticitycoordinates like e.g. then u′v′, but the same below invention principlescan also be embodied in other color representations, e.g. linear RGBrepresentations (i.e. the calculations are then done with RGB componentsdirectly rather than Y components), etc. Also, the skilled person willunderstand which of several manners one can use to co-encode theparameters characterizing the functional mappings (which can be e.g. amultilinear function defined by its change-of-segment points), whichwill typically be as metadata in or associatable with the image signalS_im (e.g. a SEI message structure or similar), which means that at thetime a receiver needs the parameters to make sense of the encoded pixelcolor data for transforming them to e.g. a linear RGB output image forrendering on a connected display, it must have obtained those lookdefining parameters by some data communication technology, e.g. aconnectable memory location via some communication link.

This particular combination of a “sensitivity” mapping which brightensat least the most dark object colors in an image (in a subrange ofdarkest pixel colors, which one can in practice define to be the 10%lowest luminances of a normalized linear image, which may be e.g. a Ychannel, or the corresponding lowest linear RGB values) and agamma/power function works well to handle the colorimetriccharacteristics of HDR images, and in particular their typical mismatchwith LDR rendering. One could of course envisage many LDR renderings,e.g. the trivial one which just ignores all colors which are consideredtoo bright or dark by clipping, but that is not necessarily the best LDRlook, and certainly not usable for good quality HDR look reconstruction.

As both an HDR image (directly in mode i) and an LDR image (inparticular an LDR image which is actually encoding an HDR image) can beactually encoded in a similar e.g. 3×10 bit HEVC container, one mightask which is the difference then between a HDR and LDR image. Thisdifference is a colorimetric difference, which is very important when adisplay, viewing environment and viewer are involved. Mathematically onecan measure it from the typical pixel values in the normalized image(i.e. a normalized LDR or HDR image), and in particular the histogram.If it is a regular LDR image, typically there will not be such a highcontrast between the darkest and the brightest pixel colors. Of course,also in an LDR images there may be values which are white, as well asvalues which are black (zero), but these will correspond to differentactual to be rendered luminances, because there is a different codeallocation function defining them. Our novel receivers/decoders willrecognize the situation and apply in each case the appropriate decoding.When we say the LDR image should be directly usable on a legacy display,we mean that the receiver supplying decoded images to the receiver willunderstand/decode the luma values with a legacy code allocationfunction, i.e. typically the gamma 2.2 of Rec. 709. Now a master_HDR(mode i) image may be encoded by a totally different code allocationfunction, which means that a black luma of say 0.05 corresponds to amuch darker black than for an LDR image, and 0.96 corresponds to a muchbrighter color. In addition to that mode ii introduces a further newconcept, that the luma codes may be now related to the HDR to berendered lumas by yet another code allocation, and which may even bevariable depending on choices by the grader (in particular the customcurve)! In particular, a mode i image will typically have not arelatively uniform (well-lit) histogram as in legacy LDR encoding, buttypically a bimodal histogram, in which there are a number of darkerobjects, and a number of much brighter pixels (e.g. the outside pixelswhich would be 100× brighter in a linear luminance representation, butmay be e.g. 3× brighter when using a particular code allocationfunction, i.e. in the ultimately used luma representation). In some HDRimages the brighter pixels may also be few in number, e.g. a couple oflamps in a night scene. In a mode ii image, the relation will again bedifferent. There will still be some sufficient difference between thebright and dark regions (assuming in the simple elucidation here thatthe HDR images is so formed), not only because then relatively simplefunctions can map to the Rec_HDR, but also because even the LDR directrendering one may desire to retain somewhat of the contrast look. But onthe other hand the two luminance ranges may have been shrunk towards orinto each other to a certain extent because of the limitations of theLDR gamut. But what is important in all this is that one can still seesome signature of whether an image was of a LDR or HDR scene. Not onlymathematical image analysis algorithms can analyze the dynamic rangelook as encoded in images (e.g. for real-time television production inwhich the ultimate quality of the looks is less important than e.g.production costs), for which an image analysis unit 177 can be used. Butin general our coding technologies in their most high quality formatwill be used with a human color grader at the creation end, who can seeon typically a HDR and LDR display how the system behaves (i.e. what theLDR and HDR looks actually look like), turn the dials of his gradingkeyboard, and ultimately encode an LDR image and HDR reconstructionfunctions he is happy with. Note that typically receivers need not do afull analysis of the situation. They need not per se care about whetherthe normalized image they received was a HDR image or an LDR image, andwhich LDR image variant. They just need to “blindly” apply the functionsthey receive. The only thing they need to typically know is what thefunctions define and/or what the sole image defines. So typically thesignal will contain an indicator (IND) what type of signal it is. Ofcourse there may be communicated many things about the signal, e.g. fortelevisions which want to do their own smart further image improvement,but typically what a receiver needs to know minimally is that this HDRencoding signal S_im is of the type which contains an image directlyusable for LDR rendering (whether its look can be fine tuned into abetter LDR image by receivers which can tone map the received LDR imageLDR_t with further tone mapping functions [parameterized with Ff1, Ff2,etc.] or not). With this information the receiver knows that if aconnected LDR display is to be served with the appropriate images, theLDR look images can be directly sent to it, and if it is a HDR display,first the color transformations will be applied to obtain the correctHDR images for rendering. The skilled reader will understand that thiscan be indicated in several ways, e.g. with a keyword like “DIRECTLDR”,or with a set of numbers “100/5000”, which indicate that the sole imageis an image intended for a 100 nit display (actual or reference display)and was derived from and is mappable to a 5000 nit HDR image (notmeaning that not other images for displays of other peak brightness canbe derived from the LDR image with the information in the parametersdefining the color transformation functions), etc.

If we now look a little more in detail to what a HDR image may typicallybe (when normalized and to be graded to an optimal mode ii LDR image),one should understand how various scenes will typically be master gradedin a HDR reference display defined environment with a peak brightness ofe.g. 5000 or 10000 nit.

Again taking the elucidating example of an indoors scene with a brightsunny outdoors, one may want to grade the outdoor colors in the M_HDR toapproximately HDR middle grey, so about 20% of 5000 nit, i.e. +−1000nit. The indoor colors should not be rendered with actual typicalindoors luminances, since we are watching the movie in anotherenvironment, e.g. a typical dim environment of television viewing. Sodefinitely the indoor colors should not be rendered at 1/100^(th) of thesunny outdoors pixel luminances, since those are also not renderedexactly, only an exact copy at any receiving side of what the referencemaster grading on the reference display would have been. We need to takeappearance to the adapted average viewer into account, in particularthat in the HDR look the indoors should not look unrealistically dark.We can grade those colors at e.g. 1/10^(th) the average luminance of the“sunny outside” image region colors, so around +−100 nit. However, nownaively mapping those lumas onto an 100 nit LDR reference display (witha color mapping which is say close to a linear stretch at leastconceptually), the sunny outside colors in LDR would look perfect,around about 20 nit and upwards to white, but the inside colors would berendered around 2 nit, which would be seen as psychovisual blacks. Thisis why one needs to do “some” optimization, which might be quite complexdepending on the complexity of a particular HDR scene, which is whypreferably have a human color grader involved at the content creationside, for the aspects of our encoding framework. To make those indoorscolors also reasonably viewable, we may put them somewhat darker thanmiddle grey (18%), but not to much in the optimization. So we may wantto boost those darker colors with a factor typically between 5 and 7(depending on what is in the dark respectively bright subregions ofcourse, the optimization may be different for a dark basement in whichone should barely see tools on the wall and may clip the light of thelamp illuminating it), keeping the brighter colors above that. FIG. 5shows two example scenarios of our HDR/LDR encoding chain. Curve 501 and502 show only typical first (“sensitivity”) tone mapping curves, i.e.before the gamma. They are defined by

${v = \frac{\log( {1 + {( {{RHO} - 1} )*{xg}}} )}{\log({RHO})}},$with possible normalized values for the input xg between zero and 1.0,and an optimal RHO value in case the master HDR was defined with areference display peak brightness of 1000 nit for curve 501 (meaningthat whatever content was in the captured scene, the object luminancesin the M_HDR are defined between zero and maximally 1000 nit, the valueto which e.g. a welding spark or the sun may be graded), and 5000 nitfor curve 502. The optimal RHO value can be determined in many ways asthe skilled reader will understand. E.g. the grader may chose it,appropriate for what he considers a good LDR look given the specificM_HDR image. Or, an apparatus at the creation side may automaticallycalculate it e.g. according to the following equation:

${RHO} = {{power}( {33,{( \frac{\log( {1 + {( {33 - 1} )*{{power}( {( \frac{{PB}_{HDR}}{10000} ),\frac{1}{GAM}} )}}} )}{\log(33)} ).}} }$

In this equation PB_(HDR) is the peak brightness of the referencedisplay associated with the M_HDR grading (i.e. which defines the rangeof possible values, and typically corresponds to the PB of a realdisplay on which the grader studied and created his master HDR look),e.g. 1000 or 5000 nit as in FIG. 5, and GAM is a gamma value, which maytypically be e.g. 2.4. Of course the apparatus (or grader) may deviatefrom these values by any other algorithm or heurisitc, e.g. in case asomewhat brighter, or flatter, look is required, etc.

Now one can see in FIG. 5 that if one determines the boost factors(compared to the diagonal, the normalized HDR luma being on the x-axis,and the normalized LDR luma on the y-axis) for the first/sensitivitytone mapping part only to a value between +−1.5 and 4.0, one gets afterapplying the gamma mapping with a gamma of 2.4 also, boost factors ofaround 6-7 for the darkest 10% of colors (curves 503 resp. 504 being thecombined mapping of log and gamma), which is roughly what one needs (thegrader can later fine-tune as desired with his arbitrary tone mappingcurve, but this is a good strategy for e.g. auto conversion apparatuses,which minimally involve the grader only in case fine-tuning is needed ordesirable). In general one would like to generically have a boost of+−4-8 for the combined log/gamma tone mapping operations (i.e. unit 602and 603), which would mean that a boost value between 1.5 and 5.0 wouldbe appropriate for the RHO-based sensitivity part only (unit 603). Anytone mapping function for unit 603 having such a behavior for the darkercolors would satisfy what we need for our invention, but the abovelog-based equation is a simple pragmatic manner to realize this. Thebehaviour for the lighter colors above will typically be a gentlecompression, i.e. with a function shape which typically non-linearlymaps the lighter luminances above the range taken up by the boosteddarker colors. Now one can have very complex HDR images, which mightdesire other values, but such extreme situations can be handled by anappropriate arbitrary curve definition by the grader (or anautomatically grading algorithm). Note that at the decoding side, thechain of processing needs to be substantially invertible, to be able tocalculate Rec_HDR from the sole communicated LDR image(s). Substantiallyinvertible means that we do not necessarily have to obtain exactly thesame color component values in Rec_HDR as in the original M_HDR, but thecolor differences should be within a tolerance limit. Therefor thereceiver should ultimately be able to obtain the needed colortransformation functions for upgrading to the HDR look Rec_HDR, whetherhe calculates them by inverting the downgrading functions originallyused at the receiver side when making the LDR_o (or LDR_i) form M_HDRand receiving the shape information of those functions, or by directlyreceiving the inverse functions needed for doing the upgrading toRec_HDR. This will inter alia typically mean that for the arbitrary tonemapping function which the grader can define to fine-tune the LDR lookto his strict preferences, he will need to define a monotonicallyincreasing function relating the normalized LDR and HDR lumas as theskilled person will understand.

The basic mode ii technical chain may work in a simple manner. E.g. forsome less critical scenes, the grader may fill the arbitrary functionwith default values being an identity transform. Note also that althoughwe describe the basic technical components necessary in the chain, inpractical realizations one or more of these blocks may be grouped inactual units performing the functions. E.g. in some applications it maybe desirable to send a total LUT of all color mapping functionstogether, while in other applications it may be advantageous to send theseparate functions, because the television (automatically, e.g. afteranalysis of the scene, or under user interface control by the viewer)may e.g. want to further tune e.g. the first function which brightensthe image somewhat compared to the sensitivity or RHO value received viathe image/video communication technology. More advanced versions may usesome further processing steps, e.g. the method of encoding may determinea gain value (gai) for mapping the maximum luma of the lower dynamicrange image (LDR_o) to a specific value of the possible values in thereconstructed high dynamic range image (Rec_HDR), and encoding that gainvalue in the image signal (S_im), which should not be confused with thefinal scaling form normalized colors to the peak brightness of theconnected display (e.g. Lm=5000 nit). This gain allows more versatilegrading and/or coding.

A very useful improved method of encoding a high dynamic range image(M_HDR) comprises: after applying any of the above color mappings todetermine the lower dynamic range image (LDR_o), applying a furthertechnical tone mapping (301) to determine a second lower dynamic rangeimage (LDR_i) which can be used to drive LDR displays as an alternativedriving image alternative to the lower luminance dynamic range image(LDR_o), which technical tone mapping is determined by: a) determining afirst geometrical region of the lower luminance dynamic range image(LDR_o) for which the visibility of banding in the correspondingreconstructed high dynamic range image (Rec_HDR) is above an acceptablelevel, b) determine a range of lumas (L_u) for that region, c) determinea second range of pixel lumas (L_uu) adjacent on the luma axis to therange of lumas (L_u), wherein the second range is identified to fulfillthe conditions that it has a number of lumas above a minimum number(MIN), and corresponds to a second geometrical image region whichcontains a texture which can be represented using less than the minimumnumber of codes in an LDR image (LDR_i) upon which to apply thefunctions yielding a reconstructed high dynamic range image (Rec_HDR) ofsufficient visual quality for that second region, and d) determining aredistribution mapping function which redistributes the lumas of thefirst and second range of lumas, so that additional codes are availablefor the first range, and outputting in the image signal (S_im) valuesencoding the function shape of the redistribution mapping function orpreferably its inverse.

One is somewhat limited in the trade-off between full or sufficientlyprecise reconstruction of the Rec_HDR, and the look of the LDR imageLDR_o, in particular if hardware (and grading cost) dictates that arelatively limited amount of grading functions should be used. Some HDRscenes may not be so difficult (e.g. the average viewer may not be toocritical about whether the shadows of a shadow side of a sunlit streetare a little darker, or a little more light grey, as long as thedeviations from the optimal look are not too excessive), but some HDRscenes can be more critical (e.g. somewhere on the HDR luminance rangethere may be a guy partially hidden in luminous mist, and if the localcontrast there is too high he may only be somewhat too visible, but ifthe contrast is too low, he may be invisible, changing the story). Itwould be advantageous to have another dimension of grading possible, atleast for receivers which are not legacy (and don't know how to do anyHDR processing), and can do some further tone mapping. A legacy displaycould then get the “best effort” LDR image, which will be the sole imagetransmitted, but smart future receivers could do some smart technicaltricks to further optimize the LDR look, so that it comes closer to whatthe grader desires (maybe even clip some values in the ultimate LDRlook, which would be incongruent with HDR reconstruction if thathappened in the sole transmitted LDR image). Having such a possibility,some encoding methods or encoders could cater for this. Squeezing a verycomplex very high contrast ratio HDR image in an LDR sole image (e.g. anHDR image which have several important regions with many grey values,e.g. a dark unlit room an relatively well-lit second room, and at thesame time a colorful sunlit outside, and with these 3 regions alsocontaining important gradients covering many grey values, e.g. of awhite table under the lamp in the well-lit room), it could happen thatone or more regions become unacceptable, because due to the limited wordlength (e.g. 10 bit) for the color components, somewhere (depending onthe shapes of the color mapping functions) there is banding which isjudged too severe. This region can be identified, e.g. by the graderspotting it (and he may be aided by being pointed to potentiallycritical areas by HDR image analysis software in the grading apparatus).Banding detectors can calculate e.g. that for an extended region(potentially also taking into account which luminances this region has,and estimated JNDs) there are jumps of each time a number ofsuccessively equal colors, and they can define an acceptable level basedon the values from such a calculation (and typical in factoryexperiments). The grading apparatus after having found such a region(e.g. by finer segmenting what the grader has roughly selected), canthen roughly determine the range L_u of luminances corresponding to it.E.g. there may be banding in a blue sky, the colors of which haveluminances between L_sky_low and L_sky_high. The problem would bemitigated, if the LDR encoding had more values to encode the image in,where we should understand that at the encoding side the M_HDR and anytransformations may still be of very high precision. But these codesdon't exist: we only have the 10 bits available for all neededluminances, and we also need to sufficiently encode all other imageregions of different illumination. But a trick can be used, if somecodes can be borrowed from regions of the image which have luminancesadjacent to L_u, especially if the visual quality of those regionsdegrades little by taking a few codes from their code range (whichtypically the grader will judge, by a simple operation of accepting theresult, or disagreeing in which case another attempt will be tried,which is more aggressive in case the banding is still to high for theoriginal banded area and the adjacent region can still be deterioratedmore, or a little less aggressive if the grader indicates that theadjacent region starts deteriorating too much). A simple manner toredistribute codes is e.g. a linear or non-linear modification of thelocal function part. Now the issue with the sole transmitted imageLDR_o, is that the sky may e.g. have become a little too dark, and maybetoo contrasty by this operation (and also the adjacent regions may besomewhat too dark, and their texture look may have changed etc.). Thismay be not too problematic in case of small changes and less criticalscenes, and a little more inconvenient for difficult scenes. It is theprice legacy systems may have to pay because they can do absolutelynothing with any of the received data except for directly rendering theLDR_o, but new receivers can apply the inverse of the transformationsused to redistribute the lumas, to create an LDR look very close to theoriginally intended one (i.e. with the appropriate sky luminances etc.),but now with less banding. A receiver need not do much smart analysis,it only needs to see that such a technical tone mapping function isavailable, and apply it to the reconstruction of the sole transmittedLDR image LDR_t to obtain the better LDR look image LDR_ul. A number ofmethods may also be applied in the grading apparatuses to come to goodsuggestions for the adjacent region, e.g. a region with a sufficientamount of lumas (e.g. equal to the amount in the sky) and with somecomplex texture may be determined. Simple embodiments may e.g. use allcodes below the range of the banded regions, up to the blackest black.

The amount of additional codes for the first range is determined basedon a banding visibility criterion for the first geometrical region. Anautomatic algorithm may come with a proposition, e.g. 20% additionalcodes, and typically the human grader will acknowledge this. Thealgorithm may also highlight the regions it had to deteriorate, e.g. byblinking a coloring of those regions, so that the grader can quicklycheck whether those are of sufficient visual quality also in thereconstructed HDR Rec_HDR.

In most practical embodiments, the identification of the firstgeometrical region showing the excessive banding is typically ultimatelyperformed by a human grader via a user interface unit (105), e.g. byscribbling a wavy line along the banded region, and the amount ofbanding of the first geometrical region in the reconstructed highdynamic range image (Rec_HDR), and the visual quality of reconstructionof the second geometrical region in the reconstructed high dynamic rangeimage (Rec_HDR) are judged by the human grader as acceptable orunacceptable, wherein in case of the acceptable judgement the valuesencoding the function shape of the redistribution mapping function orits inverse are encoded in the image signal, or in case of theinacceptable judgement the steps are done again with differentparameters to come to an alternative redistribution mapping function.E.g. 10% more codes may be allocated to the banded region, perhaps atthe expense of an enlarged adjacent luma range L_uu.

An interesting embodiment of the method of encoding a high dynamic rangeimage (M_HDR) has the pixel colors of the lower luminance dynamic rangeimage (LDR_o) are encoded as a luma channel and u′ and v′ colorcoordinates which are calculated as

${u^{\prime} = \frac{4X}{X + {15Y} + {3Z}}},{and}$${v^{\prime} = \frac{9Y}{X + {15Y} + {3Z}}},$with X, Y and Z being the device independent 1931 CIE color coordinateswhich are derivable for any RGB representation (i.e. the and CIE 1976(u′,v′) chromaticity representation). Normally according to the legacyphilosophy, images (especially LDR images) would be encoded as YCrCbimages. But if one changes any codec (e.g. for internet transmission),one might as well encode the color components as Yuv component planes,which has some advantages both in image quality of the transmittedimages, and ease of applying the various color transformations of oursystem (of course the legacy televisions will not then be able to makegood looking pictures out of this).

We have found a generically chosen luma definition (defined by thechosen full tone mapping strategy of the above steps, ultimatelyobtaining lumas in LDR_o or LDR_i) which together with twoluma-independent chromaticity coordinates, in particular the u′,v′coordinates standardized by the CIE, will be a good codification to beused in standard image or video compression technologies. Compressiontechnologies similar to e.g. HEVC will typically apply at least spatialcompression by doing DCTs of blocks of samples, but for video they mayalso do motion estimation based compression, etc.

In simple embodiments the encoding of the functional colortransformation behavior of the color conversions can be communicatedwith only a few simple parameters by storing in the metadata associatedor associatable with the sole image(s): a) a sensitivity value (e.g.RHO, or an equivalent parameter defining RHO called SENS and definedherebelow, or any function or correlate of RHO allowing to determine aRHO value), b) a gamma value (GAM), and c) a number of valuescharacterizing the functional shape of the arbitrary function mappingthe lumas.

A method of encoding a high dynamic range image (M_HDR) comprisingdetermining a gain value for mapping the maximum luma of the lowerdynamic range image (LDR_o) to a specific value of the possible valuesin the reconstructed high dynamic range image (Rec_HDR), and encodingthat gain value in the image signal (S_im). This is useful for a furtherscaling of the Rec_HDR image obtainable from the LDR image, e.g. if theLDR image of a relatively dark shot is represented relatively brightly,i.e. which lumas up to a relatively high value in the range of possibleLDR lumas, yet the HDR image should be rendered not too brightly, whichis handled best by already decoding it with a not too high maximum luma.

A method of encoding a high dynamic range image (M_HDR) comprisingdetermining a saturation modification strategy either of the colors ofthe high dynamic range image (M_HDR) to colors in the lower dynamicrange image (LDR_o), or the other way around, and coding this saturationmodification strategy as parametric values in metadata in the signal(S_im). Typically graders will also want to affect the saturation of animage, e.g. they may change the saturation of the LDR_o obtained fromM_HDR with some saturation mapping strategy, and/or the saturation ofRec_HDR from LDR_o (e.g. first a tone mapping leaving the u,vchromaticities of the obtained Rec_HDR colors at the values they had inLDR_o, and then changing the saturations of those Rec_HDR colors).

Some embodiments of the method of encoding a high dynamic range image(M_HDR) comprise: after applying any of the above color mappings todetermine the lower dynamic range image (LDR_o), applying a furthertechnical tone mapping (301) to determine a second lower dynamic rangeimage with redistributed lumas, i.e. lumas of typically slightly changedvalues for at least a geometrical region and luma subrange of the image(i.e. a redistributed luma low dynamic range image LDR_i), whichguarantees that at least in the to the grader more important regions ofthe second lower dynamic range image (LDR_i), e.g. which are carefullywatched by the typical intended viewer, because they are e.g. large andbright, and prone to banding, sufficient luma codes can be allocated toencode the textures in those regions with sufficient precision to enablereconstructing the reconstructed high dynamic range image (Rec_HDR) witherrors below a predetermined error criterion (a minimal banding amount).

It is important for some embodiments that one doesn't choose any weirdtone mapping strategy. In particular, if one wants to be able to obtaina good quality Rec_HDR, i.e. close in mathematic pixel color values toM_HDR, then one needs to assure that there are no undersampled texturesin LDR_i, which happens if one assures that the final mapping beforeuniform quantization is nowhere too flat.

Typically this may be done by luma counting strategies on the LDRpicture and/or luma counting strategies such as e.g. a banding detectoron the HDR image, or any such predetermined HDR reconstruction errorcriterion. In some embodiments the criterion may be performed by a humangrader. Its presence can be seen by having a technical remappingstrategy co-encoded in S_im, to be applied by the smarter futuregeneration receivers.

The method may be embodied in an image encoder (100) arranged to encodea high dynamic range image (M_HDR), comprising:

-   -   A dynamic range conversion unit (104) arranged to convert the        high dynamic range image to an image of lower luminance dynamic        range (LDR_o), the dynamic range conversion unit (104)        comprising connected in processing order: a) a normalizer (601)        arranged to normalize the high dynamic range image to a luma        axis ranging over [0,1] and to output normalized luminances        (Yn_HDR), b) a gamma conversion unit (602) arranged to apply a        gamma function to the normalized luminances and to output        gamma-converted luminances (xg), c) a first tone mapping unit        (603) arranged to apply a first tone mapping which boosts those        gamma-converted luminances which lie below 0.1 with a        predetermined amount lying between 1.5 and 5.0, yielding lumas        (v), d) an arbitrary tone mapping unit (604) arranged to d)        apply an arbitrary function which maps the lumas (v) to output        lumas (Yn_LDR) of the lower dynamic range image (LDR_o); and the        image encoder (100) further comprising:    -   an image compressor (108) arranged to apply a data reduction        transformation to the colors of the lower dynamic range image        (LDR_o) which colors are organized in component images, and        which reduction transformation involves at least applying a DCT        transform to blocks of adjacent color component values, yielding        a compressed codification (LDR_c) of the pixel colors of the        lower luminance dynamic range image; and    -   a formatter (110) arranged to output in an image signal (S_im)        the compressed codification (LDR_c), and arranged to in addition        output in the image signal (S_im) values encoding the function        shape of the color conversions as metadata, or values for their        inverse functions, which metadata allows a receiver to        reconstruct a high dynamic range image (Rec_HDR) based upon the        lower luminance dynamic range image (LDR_o).

A pragmatic embodiment of such an encoder is one in which the gammaconversion unit (602) uses a gamma value equal to 1/(2.4), and/or thefirst tone mapping unit (603) uses a tone mapping defined by theequation

${v = \frac{\log( {1 + {( {{RHO} - 1} )*{xg}}} )}{\log({RHO})}},$with RHO having a predetermined value, which value typically is afunction of the peak brightness of the intended serviced display and/orthe reference display associated with the master HDR encoding M_HDR.

An image encoder (100) arranged to specify a gain allowing mapping themaximum of the luma codes in the lower dynamic range image (LDR_o) to achosen luma value of the reconstructed high dynamic range image(Rec_HDR), and having the formatter (110) arranged to output this gainas a value in metadata in the image signal (S_im).

An image encoder (100) as claimed in any of the above encoder claimscomprising a technical tone mapping unit (106), arranged toautomatically or human-guided determine texture and statisticsinformation of the lower dynamic range image (LDR_o), and in particularat least one critical geometrical region which is prone toreconstruction errors in particular banding in the Rec_HDR, and on thebasis thereof calculate a second tone mapping (Ff1, Ff2, . . . ) forbeing applied as a transformation to the lower dynamic range image(LDR_o) to yield a second lower dynamic range image (LDR_i) having aminimal number of luma codes (e.g. 1.3*L_u) characterizing the texturesof at least some important, error-prone regions of the second lowerdynamic range image (LDR_i), thereby allowing reconstructing thereconstructed high dynamic range image (Rec_HDR) with errors below apredetermined error criterion. To enable communicating the necessaryinformation allowing after encoding a receiver to mirror-wise implementour mode ii system, it is useful to transmit (or store for latertransmission) a high dynamic range image signal (S_im) comprising:

-   -   A pixellized lower dynamic range image (LDR_o) with encoded        pixel colors; and further:    -   a sensitivity value (RHO); and    -   a gamma value (GAM); and    -   a gain value (GAI); and    -   a set of values specifying an arbitrary tone mapping function        shape (P_CC) . . . .

From these values the receiver can then determine the function shapes ofall functions to be applied to the sole communicated LDR image (LDR_o,or LDR_i), should any image of higher dynamic range than the 100 nit LDRimage be required and calculated.

In particular the S_im may also comprise values 207 codifying atechnical remapping strategy (Ff1, Ff2, . . . ) for mapping between anartistic LDR grading as desired by the human creator/grader of thecontent, and a technical LDR which when sampled has sufficient lumas forall regions of the image for good Rec_HDR reconstruction, or at leastthose regions determined as more critical by an automatic image analysisunit and or a human.

In particular it is useful, because very pragmatic for receivers toquickly determine which of the now several (very) different possible HDRimage encoding mechanisms is used, in particular by comprising in theimage signal S_im an indicator (IND) specifying that an image of highdynamic range has been encoded in it, and with a method which encodesthis as an image of low dynamic range, which is directly usable, withouta need for further tone mapping, for rendering on a LDR display. Varioussuch ways of encoding can be divised and agreed, as long as any receiverunderstands it.

A memory product such as a blu-ray disk storing any embodiment of ourhigh dynamic range image signal (S_im).

To have an image communication chain, on the receiving end one may havevarious realizations of apparatuses being or comprising an image decoder(150) arranged to receive a high dynamic range image signal (S_im) andcomprising:

-   -   a deformatter (151) arranged to obtain a compressed pixellized        lower dynamic range image (LDR_c) and parameter data (P) out of        the image signal (S_im); and    -   a decompressor (152) arranged to apply at least an inverse DCT        transform to the compressed pixellized lower dynamic range image        (LDR_c) to obtain a pixellized lower dynamic range image        (LDR_t); and a dynamic range conversion unit (153) arranged to        transform the lower dynamic range image (LDR_t) into a        reconstructed high dynamic range image (Rec_HDR), wherein the        dynamic range conversion unit (153) comprises in processing        order: a) an arbitrary tone mapping unit (402) arranged to apply        an arbitrary tone mapping, the parameters which define it (P_CC)        being received in the parameter data (P), b) a first tone        mapping unit (403) arranged to apply a mapping as defined by at        least one received parameter (RHO) defining the first tone        mapping which was previously determined by any of our encoder or        encoding method embodiments, and c) a gamma conversion unit        (404) arranged to apply a gamma mapping with a received gamma        value (GAM).

This decoder will first undo all the typical legacy e.g. HEVC or similarcompression codifications, and then apply the various mappings inreverse order (note that not everything need in all embodiments beexactly in the reverse order; e.g. in Yu′v′ one may chose todoorthogonal luma and saturation processing in a reverse order, be itmaybe with slightly different mathematical functions, as long as thefinal result is exactly or approximately the intended color). Note alsothat there may be additional processing steps, which may only exist atthe receiving end (e.g. an image may be encoded in some RGBrepresentation like Rec. 2020, but may need to be converted to anotherformat as understood by a television, e.g. DCI-P3, and further convertedto the actual primaries of the TV).

So the image decoder (150) will comprising a dynamic range conversionunit (153) arranged to transform the lower dynamic range image (LDR_t)into a reconstructed high dynamic range image (Rec_HDR), and there maytypically be logic units and further color processing functions definingat least when to do what (e.g. depending on which display or displaysare currently connected and served).

A pragmatic embodiment of the image decoder has the first tone mappingunit (403) arranged to apply a function of the form:

${{xg} = \frac{( {{{power}( {{RHO},v} )} - 1} )}{( {{RHO} - 1} )}},$in which v is a pixel luma, and RHO is a real-valued or integerparameter received in the parameter data (P).

A useful embodiment of the image decoder (150) comprises a toneremapping unit (159) arranged to apply a further tone mapping (Ff1, Ff2,. . . ) received in the image signal (S_im) to the lower dynamic rangeimage (LDR_t) to obtain a second lower dynamic range image (LDR_ul)which reverses a code redistribution action, applied by any of theencoder methods 5 to 7 yielding a second low dynamic range image (LDR_i)with redistributed lumas for obtaining a reduced banding in at least aregion of the reconstructed high dynamic range image (Rec_HDR). In factthe encoder need not necessarily know exactly how any encoder came to aparticular transformation function redistributing the lumas, it justneeds to apply the inverse functions to come to substantially theintended LDR look (LDR_ul).

Another useful embodiment of the decoder can understand Yu′v′ encodingsof the LDR image, and thereto comprises a color transformation unit(155) arranged to convert an Yu′v′ color representation in a RGB colorrepresentation. Tone mapping can be done before the conversion is done,therefore leaving the conversion to RGB to the last part of theprocessing chain, or alternatively, the conversion can be done first,and an equivalent color processing can be done on RGB signals.

Corresponding to any of the decoders correspond methods of decoding ahigh dynamic range image signal (S_im) comprising obtaining areconstructed high dynamic range image (Rec_HDR) by applying the colorconversions encoded in the parameter data (P) to the lower dynamic rangeimage (LDR_t), in particular a method of decoding a high dynamic rangeimage signal (S_im) comprising:

-   -   obtaining a compressed pixellized lower dynamic range image        (LDR_c) and parameter data (P) out of the image signal (S_im);        decompressing the compressed pixellized lower dynamic range        image (LDR_c) by applying at least an inverse DCT transform to        the compressed pixellized lower dynamic range image (LDR_c) to        obtain a pixellized lower dynamic range image (LDR_t); and        transforming the lower dynamic range image (LDR_t) into a        reconstructed high dynamic range image (Rec_HDR), by: a)        applying an arbitrary tone mapping, the parameters which define        it (P_CC) being received in the parameter data (P), b) applying        a mapping as defined by at least one received parameter (RHO)        defining the first tone mapping which was previously determined        by any of our encoder or encoding method embodiments, and c)        applying a gamma mapping with a received gamma value (GAM),        which is preferably equal to 2.4. We describe a system which        allows the grader to simply yet powerfully optimize the look of        an LDR look on a HDR image of a HDR scene. Preferably as little        visual quality sacrifice is done as possible, but since LDR may        need some optimization due to its dynamic range limitations, the        system allows the grader to fine-tune micro-contrasts of        particular to him interesting scene objects, i.e. typically        important characteristic objects in this scene, and thereby if        some brightness-quality sacrificing needs to be done, sacrifice        the precise look of some less important objects like a wall in        the background, rather than the main object in the scene. The        invention can be realized in many other (partial) ways like with        intermediates containing the core technical requirements of the        various embodiments like the defining parameters embodied in        signals, and many applications of it may result, like various        ways to communicate, use, color transform, etc. the various        possible signals, and various ways to incorporate the various        hardware components, or use the various methods, in consumer or        professional systems. Any component can of course be realized in        or as a small component, or vice versa as a key core of a large        apparatus or system which predominantly functions because of        this component.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the method and apparatus according to theinvention will be apparent from and elucidated with reference to theimplementations and embodiments described hereinafter, and withreference to the accompanying drawings, which serve merely asnon-limiting specific illustrations exemplifying the more generalconcept.

FIG. 1 schematically shows an example of an embodiment of an encoder anda decoder according to our invention in an image communicationtechnology;

FIG. 2 schematically shows an embodiment of what an HDR image signalS_im according to our invention may look like;

FIG. 3 schematically elucidates by one species how one can genericallyobtain a technical LDR grading, which could even happen under the hoodautomatically without bothering the grader or content creator in someembodiments, which allows better sampling of the object lumas, andtherefore better quality reconstruction of Rec_HDR;

FIG. 4 is a simple schematical elucidation of a possible decoderaccording to our invention;

FIG. 5 shows two×two possible embodiment curves for doing thesensitivity mapping which significantly brightens the colors, or thecombined initial LDR grading, with in addition the gamma behavior; and

FIG. 6 is a simple schematical elucidation of a possible dynamic rangeconversion unit of an encoder.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 describes an exemplary typical system embodying our invention,with an image (or video) encoder 100 at a creation side, and an imagedecoder 150. We assume there is a memory 101 in a grading system whichcontains a master graded HDR look image (M_HDR) which has been graded asdesired by the content creator according to currently known colorgrading techniques for say a movie in a color grading software like e.g.Da Vinci's (similar other systems may benefit from the teachings in ourpresent application, e.g. M_HDR may come directly from a camera, aftere.g. a tuning of a camera look curve on the dials of the camera, etc.).In this M_HDR e.g. the brightness of light shining through windows mayhave been chosen to give a most pleasing look on the [0,5000] nitreference display by giving those pixels an intended to be renderedluminance L_out and a corresponding luma code v_HDR, and many furtherlight effects may have been designed, as well as other coloroptimizations. M_HDR is inputted via image input 115 in our encoder 100,and may also be looked at on a HDR reference display 102 (which exactlythe characteristics of the theoretical e.g. [0-5000] nit referencedisplay we propose for HDR encoding). This means when the grader wantsto make an LDR-look (which should not only encode the object texturessufficiently precisely so that on the receiving side a reasonablyaccurate reconstruction Rec_HDR of the M_HDR can be obtained, but alsothis LDR-look should be suitable for optimally rendering the encoded HDRscene on an LDR display), the grader can compare at the same time howmuch the LDR look given technical limitations looks on LDR display 103similar to the M_HDR, and optimize by changing the color mappingfunctions for obtaining it from M_HDR as desired according to hisliking. The two displays may be in their different optimal viewingenvironments and the grader may be looking at both separated by e.g. awall (e.g. in two enclosed reference environments with their respectivewindow opening for looking simultaneously into them, and with curtainswhich can be closed if the grader wants to see only one of them duringsome time interval). The grader may also check the reconstructed gradingof the HDR look on HDR display 102 (e.g. toggle Rec_HDR and M_HDRalternatively).

By means of a user interface unit 105 which offers the grader classicalcontrols like e.g. turning wheels or similarly sliders for settingvalues like a gamma or sensitivity value, the grader can makecolorimetric transformations defining how the M_HDR should be mapped tothe LDR look image, with the parameters of the transformations to beoutputted in an image signal S_im via an output 116 of the encoder whichmay be connectable to any image transmission medium 140, e.g. acommunication network, or a physical carrier memory like a BD or solidstate memory, etc.

The LDR look is generated via a dynamic range conversion unit 104, whichis arranged to apply colorimetric transformations on at least the lumasof the pixel colors, but also typically on the chromaticity coordinates.By lumas we mean any encoding which is ultimately convertible in aphysical luminance, or even via psychovisual models a brightness (whichis the ultimate appearance a viewer will see when the image is renderedon a display). Note that by equivalent mathematics luma transforms canbe applied as corresponding transformations on RGB components directly.Although the ultimate goal is the correct object brightnesses(appearances) in the look, we can limit our technical discussion to thedetermination of luminances in the reference e.g. [0-5000] range, or adevice independent color space like XYZ defined by this range.Furthermore we will assume that any chromatic transformations of thecolors are done in the UCS plane of 1976 CIE Luv space, however theskilled person can understand how similarly other second and third colorcomponents may be used, with the basic components of our invention beinggenerally applicable

CIELuv defines u and v from XYZ (similarly one can transform from someRGB) as:

$u = \frac{4X}{X + {15Y} + {3Z}}$ and$v = {\frac{9Y}{X + {15Y} + {3Z}}.}$

We assume for simplicity that the HDR and LDR gamuts (i.e. the gamuts oftheoretical displays associated with the encoding mathematics of the twoimages) have the same three (or more) R, G, B, primaries, and can hence,by scaling the respective maxima of say 5000 and 100 nit to 1.0, becollocated as exactly overlapping. So a tone mapping from HDR to LDRthen becomes a relative transformation along the normalized lumadirection within this single device dependent RGB gamut. E.g. if onewants to make the darker colors in the HDR look the same on an LDR andHDR display, this becomes as a relative transformation in the same gamutthe following: because in a 5000 nit defined color definition suchcolors in the HDR image will have small codes (e.g. below 0.1) we needto brighten them to become sufficiently visible on a 100 nit LDRdisplay, e.g. with values around 0.3. The exact mapping will depend onthe definition of the lumas for both the LDR and HDR image, because as ageneralization of the “gamma 2.2” definitions of legacy LDR image andvideo encoding, we can now define arbitrary code allocation functionsmapping from physical luminances to luma codes (or the other way around,because typically tv engineers start by defining a reference displaywhich in addition to a reference [0-5000] nit range has some referencedisplay EOTF behavior indicating how the e.g. 1024 lumas map torenderable luminances along that reference range). Not only could we usea power 1/(7.0) gamma as OETF, but we could even use discontinuous codeallocation functions if in a shot of images there are no luminancespresent between a lower range of luminances and an upper range ofluminances. Also note that working in an Y′uv representation withluma-independent chromaticities (u′,v) allows us to work totallyindependently and freely in the achromatic and chromatic directions ofcolor space.

Limiting our elucidation for the skilled reader to achromatic mappingsof HDR-2-LDR only, these can be formulated generically as in principlean arbitrary tone mapping function from the [0,1] lumas of the HDR-lookimage to the [0,1] lumas of the LDR-look image, as one can see with anexample in FIG. 2 a.

Specifying such a function, we will assume that the mapping on allcolors (Y_M_HDR, u,v) is done so that for a non-achromatic color(u< >u_wp, v< >v_wp) where (u_wp, v_wp) are the chromaticity coordinatesof a chosen white point such as D65, the determined tone mappingfunction 210 is linearly scaled to a maximum luminance L_max(u,v)achievable for that color, as taught in more detail in WO2014056679. Theskilled reader may understand how such processing instead of beingapplied in a Y′uv color encoding can similarly also be done in an RGBcolor encoding.

Once the grader specifies such a tone mapping behavior, encoders havesufficient information for a brightness dynamic range transformation tobe applied on any possible color in M_HDR, yielding an original(uncompressed, possibly still unquantized in a float representation) LDRlook LDR_o. From this, any exact or approximate mathematicaltransformation can be determined by the encoder, which allows a receiverto do the prediction the other way around, from LDR_o to Rec_HDR. Thegrader can check via an image output 111 how such an image (aftersufficiently formatted into an image signal which can be communicatedover an image communication link such as e.g. HDMI) would look on areference (say 100 nit, or in the future maybe 500 nit) LDR display 103.

We will however teach in the present invention that it is useful whenthe tone mapping is not just constructed in any generic manner, but in aparticular manner, and the (few) corresponding parameters are usefullyencoded as separate metadata in the image signal S_im, because they canthen be advantageously used at a receiving side, e.g. during tunabilityto derive an optimal driving image for a particular X nit display.

As a first parameter, the grader will choose e.g. a sensitivityparameter SENS, or RHO directly. This will be a value which isintuitively similar to the ASA or ISO values known from photography, andtypically determines how bright the LDR image will appear (inter aliahow much the dark object colors of M_HDR are raised).

As a preferred embodiment the encoder can use a EOTF/OETF function whichalready provides a good initial LDR look,

which EOTF function is defined as follows:

$L = {{Lm}( \frac{\rho^{v} - 1}{\rho - 1} )}^{\gamma}$

This equation defines the to be rendered HDR luminances L correspondingto luma codes v in [0,1] spread equidistantly based on the amount ofbits available for the luma code word of the pixel colors, as say 1024possible values. Lm is a choosable variable indicating the peakbrightness of the reference display of the M_HDR or Rec-HDR linearcolor/luminance representation, which may e.g. be fixed as 5000. E.g.the grader will have dials to choose the sensitivity which may typicallybe related to rho as:

$\rho = ( {\frac{SENS}{8\sqrt{2}} - 1} )^{2}$

Together with the SENS (RHO) value determining the dark colors behaviorand some overall brightness look, the grader can co-tune gamma (GAM) assome bending parameter reallocating object/region brightnesses along therange of possible LDR lumas. Of course when mapping from luminances L ina reference XYZ space representation of the M_HDR grading (which may bea useful intermediate representation), to v luma values of the LDR look,the grader will define the inverse function.

Doing elementary mathematical calculations on the RHO division, it canbe seen that the inverse function (OETF) is: first apply a 1/(GAM)yielding

${{xg} = {{power}( {\frac{L}{Lm},{1/{GAM}}} )}},$and then calculate:

$v = {\frac{\log( {1 + {( {{RHO} - 1} )*{xg}}} )}{\log({RHO})}.}$

Typically at the encoder there may be one of various possibleembodiments of an image analysis unit 177. This unit may be arrangedwith artificial intelligence to analyze regions in the image, and whichof these regions could yield particular problems in HDR encoding, inparticular of the mode ii type. In particular, it may identify regionswhich could be prone to banding, and regions which are sufficientlytextured, so that they can be encoded with a lesser amount of lumaand/or color component codes. In some applications this unit mayautomatically come to a final encoding proposition (e.g. a transcoder)without any human grader involvement, but in other applications it maye.g. bring regions under the attention of the grader, so that he canscrutinize them. Of course there may be an interaction with the userinterface, e.g. the grader could indicate that he wants to mitigate thebanding with a particular region, or with a particular texture, and thenunit 177 can extract such a region, and its luma range, etc.

As we can see in FIG. 2, although one can decide to encode a final tonemapping function in typically a reserved LUT space in the metadata 205,typically one will encode a sensitivity parameter (e.g. 200 ISO) or aRHO value, and a gamma value (e.g. 2.8) in respectively sensitivitymetadata field 202 and gamma metadata field 203. FIG. 2 schematicallyshows how an image or video signal S_im (200) looks, and the skilledperson will of course know that can be defined in practice in the manydigital variants, given existing image data containers etc. Our encoderembodiments use a classical 3 component encoding of the pixel colorimage (201), which will be our grader-optimized LDR look image. This LDRimage LDR_o will typically be classically DCT encoded, run-lengthencoded, formatted etc. according to an image encoding standardizedformat like JPEG, or a standardized video encoding format likeMPEG-HEVC, VP1, etc. The skilled reader will understand thatreformatting colorimetrically to be able to reuse legacy (or similarfuture) coding technologies as a generic concept is part of ourinvention, but it is not so important which of such codings is actuallyused. And another part of our invention is the metadata needed to makesense of the data, e.g. at least when recovering the Rec_HDR look of thescene (because the LDR look can in theory be used directly to drive anLDR display, with no further dynamic range processing but onlycolorimetric redefinition mapping from Y′uv to some device dependent RGBspace encoding).

Furthermore, the grader can use a GAIN value (co-encoded in a gainmetadata field 204) so that the functions need not per se map 1.0 to1.0. E.g., the gain may indicate how an LDR image which is defined overthe full range [0,1] is to be mapped to only say a [0,1500] subrange ofthe [0,5000] range of the HDR display. The other way around limiting theLDR range used is in principle also possible, though less likely to beused. This gain can be used to make some images not too bright, as onecan imagine if the scene is e.g. a misty scene, or a dark image which isreasonably brightened in LDR, but needs to stay dark in HDR.

These three parameters (RHO, GAM, GAI) give already a very useful firstmapping of a M_HDR image to a corresponding LDR look image, with aroughly global brightness or illumination adjustment. This may e.g. besufficient for broadcasting real life shows, where the optimalparameters are determined right before the start of the broadcast. Morecritical users like movie producers, may want a more fine tuned controlover the look. They may want to specify a more general tone mappingfunction than the above “log gamma” one, with finely positioned bends inthe curve which can raise e.g. the average local brightness or contrastof a particular object (e.g. a face) to a desired subrange of allrenderable LDR luminances (or more precisely their corresponding lumas).Or a specification of a local slope can specify the desired contrast insome interesting subrange BL of an important region in the image, at thecost of brightness positions and contrasts of other regions/objects inthe LDR look image.

Now an important thing to understand is that with our mode-i (HDR-look)system the grader can define such mappings arbitrary, because we onlyneed to derive an LDR-look image (which is no reconstruction, but can bedone data-destructively if so desired by the grader), because in thatencoding approach we have the HDR-look image already encoded as soleimage in the image signal S-im. In mode-ii systems however we need tofulfill a dual criterion: on the one hand we need to be able toreconstruct the Rec_HDR image with good quality, but on the other handwe want sufficient freedom to create most if not all LDR looks a gradermay desire (and then can be quite creative at times, as one can see e.g.in the movie Sin City 2).

But one should understand that whatever grading LDR_o the grader hasmade with his preferred tone mapping 210, in a legacy encoding theseoutput LDR lumas will go through classical uniform quantization (andeven DCT-ing). So we should be careful not to create mappings which astoo flat over some parts of their range (i.e. the local derivativedelta_LDR_out/delta_HDR-in should not be too small, so that a minimumrequired amount of LDR luma codes is allocated to that rangedelta_HDR-in or the corresponding delta_LDR_out), because otherwise whenboosting that range in the LDR-2-HDR tone mapping, we will see artefactslike banding or excessively contrasty and visible DCT artefacts.

We could have a control mechanism with a stiffness of the local controlpoints which the user uses to change the shape of the arbitrary tonemapping, but that is unpleasant for the user, especially if implementedto harshly (of course the system can warn if the grader is wanting tomake really strange mapping curves, e.g. inversions like an N-curveshould not be made).

A useful embodiment is shown in FIG. 3, which elucidates the behavior ofa technical tone mapping unit 106, which can be used to determine asecond LDR look, alternatively usable to LDR_o by smarter receiversneeding to service an LDR display. We assume the grader has chosen hisdesired curve which gives the appropriate LDR look, which is the solidcurve in FIG. 3. If the tone mapping curve is not good, this will meanthere is at least one range which is too flat, which we assume here isthe part R-u of the brightest HDR an LDR pixels, let's say the sky ofthe scene. We need to be able to stretch that range L_u in LDR, so thatsomewhat more luma codes can be allocated, and in as non-destructive(little changing his look) a manner as possible for the grader.

This can be done when there is an adjacent range L_uu which containsmore textured object.

This is a way out of the conundrum that our look curve for getting adesired LDR look at the same time determines the quantization or numberof luma codes available for faithfully encoding the various HDR regiontextures (the sufficient faithful characterization of all textures beingin the scene being the primary goal of encoding quality in HDRencoding). Having 1024 different luma/grey levels (and millions ofcodes) should be sufficient to nicely encode all textures for humanvision, if well done. Complex objects can be encoded with relativelyfewer codes, since the eye firstly sees the coarse texture pattern, andthen not so much the precise values of the pixel colors. Only inparticular unfavourable situations can we have an issue if we havebrightness gradients for which we have used too few codes.

So there are two things when adapting a curve: the technical tonemapping unit 106 typically keeps the adaptation when needed sufficientlylocal on the luma axis, so that we don't perturb the lumas of too manyobject colors (e.g. avoid darkening critical dark regions too muchagain). A quality criterion for this example scene may be that we needto brighten the dark colors to get a good LDR look, so a local change inthe bright colors won't disturb that in any way. So tone mapping unit106 will typically redistribute the codes in some local luma subrangearound the problem area, and determine a corresponding adaptation curvefor this, which is the dotted line (this curve may follow somewhat theshape of the original curve, in its two image region encoding parts,i.e. if there was a parabolically bending local shape for the sky lumas,it may typically use a scaled, larger similarly bending parabolicsegment for the air, but that is not absolutely needed, since onlyprecision of coding is the criterion).

So we need to stretch the sky region brightness range somewhat, to haveenough codes for faithfully encoding a Rec_HDR blue sky gradient. Buthow much do we need to do that, and how far should we extend theadjustment range R_Adj?

That depends on a number of things. Of course R_adj should cover theregion where there is a problem, which will typically be a relativelyvisually simple region, such as a relatively uniform regions such as agradient in the sky (this blue gradient will exist somewhere along theLDR luma range). On the other hand we must need an adjacent region whichis sufficiently textured. In the unlikely situation that the adjacentregion is yet another smooth gradient (which could occur in syntheticimages like artificial gradient test images, in which case we will haveto be satisfied with whatever optimal luma allocation we can get, butthis does not typically occur in natural images), R_adj may becomerelatively big. In the normal situation where we soon encounter atextured range we can extend L_u with a range L_uu of a size whichdepends on how many codes we have to add, and the complexity of thetexture pattern. If we need to add only 3 codes to the sky, we need tosave 3 luma codes in L_uu, and if sufficiently textured we could do thatover a range of ay 10-15 lumas, depending on what the grader or viewerfinds/may find acceptable.

The apparatus can contain tables for that.

So the nasty problem with look-curve-dependent-luma-codification is nowlargely solved. On the one hand we don't darken the adjacent darkerobjects too severely, since we only shift the colors of L_uu a little onthe upper range by expanding our sky range L_u, but mostly we keep thelower part of L_uu the same, only sampled a little less, which is not avisually conspicuous issue anyway, because textures don't need so manycodes anyway. The stretched range of sky may be a little suboptimal, butshould normally not really be an issue, and we get an improved qualityRec_HDR in return. But all this is still only if we don't take anycounteraction at the receiving end, e.g. by a receiver which can't doany processing. Because in the decoder we can do a precompensationstrategy in tone remapping unit 159. This will then make the lumaallocation a purely technical matter outside of the concerns of theartistic intents of the grader. Because tone remapping unit 159 willapply the correction for the local stretch into a compression again,before using the resulting intended LDR look (LDR_ul), for e.g. drivingan LDR display. So in the example of the sky, where we stretched the skylower limit of L_u down into the brightnesses of objects in adjacentrange L_uu (thereby darkening those objects), tone remapping unit 159 ofa decoder 150 will apply the inverse mapping of 301 as a correction.This means that visually the sky range will have its original luma rangeL_u again, and when rendered on an LDR display the correct luminancerange, yet it has more precision because was allocated more textureencoding luma codes. Similarly in the LDR_ul look the object withadjacent brightnesses in L_uu will also have the correct non-dimmedbrightnesses, and only differ in precision because of the reduced amountof codes. And the skilled person can understand how this technique canalways in the various other possible situations improve the codingprecision in those regions of an image where needed, whilst keeping theintended LDR look LDR_ul of the grader. The only thing tone remappingunit 159 needs to be able to do is to apply a tone mapping strategy tothe decoded technical LDR_t, e.g. by means of a LUT, which may beco-encoded in the signal S_im (or partly encoded if the tone mapping canbe derived from e.g. a limited set of control points, e.g. delimitinglinear segments), and hence it should be clear why it is advantageous toencode this technical adjustment function separately (Ff1, Ff2, . . . )in S_im, because it can be used by the decoder even to come to a moredesirable LDR look LDR_ul, once it had been determined at the creationside and accepted by the grader, and communicated to a receiving side.

There will largely be two categories of encoder embodiments which willenable the above. The first one largely does all processingautomatically, and need not involve the user. Smoothness and texturedetectors will automatically categorize the various regions, and soidentify the gradient pattern in the sky and the adjacently located(i.e. on the luma range located below and/or above L_u) other texturedobjects. Various texture characterizers may be built-in to determine thecomplexity of the texture (e.g. fine-grainedness, amount of intertwinedgrey values etc.), and determine therefrom how visually conspicuousperturbations leading to less encoding lumas will be, and the therefromresulting needed L_uu range. As said, these preferences may be pre-builtin formulae determining the L_uu functionally, or with LUTs. Also insome embodiments DCT or other compression emulators may be present, e.g.which calculate the resulting decompressed LDR images LDR_d undervarious choices for R_adj and the functional tone mapping perturbationshape 301, and calculate a severity measure for the typical visibility(at normal viewing range, display size, surround brightness, etc.) ofthe banding and/or other compression artifacts. Texture analysis unit117 may be present for this, which is typically arranged to analysetextures, and in particular their visual impact, in both the original(LDR_o) and the encoded LDR_c, or in fact the decoding thereof LDR_dwhich will ultimately be present at the receiving end. In particularremappings to HDR by LDR-2-HDR color mapping unit 118 may be used toallow the grader to check visual impact if needed. If the grader wantsto check the reconstuctability of this M_HDR as Rec_HDR, he can e.g.toggle them in time on his HDR display 102, via HDR image output 119. Infact, the decoder may have several outputs (which we have shownseparate, but of course they can be routed internally to just oneoutput) 111, 112, 113, 114 to be able to check the various versions ofLDR.

A second category of encoders with technical re-grading may directlyinvolve the human grader. If he is checking the quality of the automaticalgorithms already, he may have an option to influence the results (i.e.typically semi-automatically). This should be simple for the grader, ashe may want to be more involved with the artistic determination of thelook, i.e. the placement of the object lumas, rather than technicalissues like compression artefacts (if already wanting to look at that,and although he will check one or more typical and approved scenarios,down the image communication line there may of course be furthercompressions which could have more severe artifacts).

In these encoder embodiments the user interface unit 105 will typicallyallow the grader to specify geometrical image areas which according tohim are particularly problematic areas. E.g. he may scribble through thesky, and the histogram analysis and texture analysis units will thenfocus on this part of the image when doing their analysis and technicalupdate partial tone mapping curve determination. E.g. they maysuccessively propose a strategy which adds some more luma codes at atime to the sky, until the grader is satisfied. E.g. an embodimentalgorithm of the tone mapping unit 106 may multiply this range of thegradient (banding-sensitive) object by k=e.g. 1.5, and select aneighbour range of a textured image region and compress that toL_uu−1.5*L_u. I.e. any linear or curvi-linear redistribution of thecodes in the two regions can be used. The L_uu may be selected to be atleast e.g. 3*L_u, which values are typically optimized by an apparatusdesigner on the basis of a set of representative images. If theproposition by the apparatus is good, the grader accepts it, making theencoder store the corresponding parameters in S_im, or otherwise a newiteration is started, e.g. with k=1.1*1.5.

The perturbation 301 will lead to a final tone mapping, with whichcorresponds a final technical grading LDR_i, which will be the LDR lookwhich is send into the communication system after further formattingaccording to our mode-ii HDR encoding system, and which largelycorresponds to what the grader desires as LDR look. The advantage ofgrader involvement is that he can indicate—at least with a minimum ofinvolvement—which regions are semantically more relevant. Thestatistical texture analyser may determine that few lumas (i.e. fewpixels) actually exist in a region between e.g. the dark lumas of a roomindoors, and the bright lumas of the sunny outdoors, and hence decide toapply a remapping strategy which applies few codes there (in case thedecoder remapper 159 can arbitrarily reconstruct the desired LDR look,we might even use a strong technical deformation curve which almost cutsthe entire scarcely used subrange out of the LDR_i encoding therebymaking immediately adjacent in LDR_i luma value the indoors and outsidesubranges). However, if in this small region there happens to be animportant object like somebody's face or an object which was emphasizedsomehow like an appearing object, the grader may counteract this.Several practical embodiments are possible, e.g. he may scribble in ourdraw a rectangle around this region, and then turn a dial whichincreases the amount of luma codes to be used for that region. Theskilled reader will understand there are various other user interfaceways to select a critical region or object in the image or shot, and toindicate how it should be encoded with lumas, even up to the graderdrawing or influencing the shape of the modification curve 301 itself.

The rest of our mode-ii system is as follows:

Optionally the dynamic range conversion unit may do some colorsaturation processing (e.g. since colorfulness decreases with darkeningand vice versa, the grader may want to compensate the saturation whichhas become somewhat inappropriate because of the luma tone mapping). Agood practical exemplary embodiment works with a general saturationfunction of the non-information destructive type. By this we mean thatalso this saturation function is nowhere too flat, so it can also bereversed. But in some embodiments the saturation function may only needto be applied in the LDR-2-HDR upgrading, and then it may be moreliberal. In FIG. 3 we have shown a smooth saturation from s_in to s_out,which can be encoded with a number of values S1, S2, S3 in a LUT in thesignal S_im. These may be the s_out values for equidistant s_in values(a sufficient amount to that the desired curve can be reasonablysmoothly recovered at the decoder), but this could also be e.g. functionshape control points. A desaturation function may e.g. be encoded as aline with slope smaller than 45 degrees (on a plot of s_in vs. s_out).In such a desaturation case, the image signal could just have an integeror float value for the multiplier in the metadata. We assumed in theelucidating example that s_out will be the saturation of the HDR image,and we need to boost the saturation of the now darkened darker colors ofthe scene to increase colorfulness, but the skilled person canunderstand there may be different processing variants in the samestructural encoding philosophy. We will for simplicity of elucidationassume that the saturation is performed in uv space, e.g. irrespectiveof the luma we may perform the operation s_out=s_in+MS(s_in)*s_in. TheMS(s_in) is then the multiplicative value retrievable from the functionas seen in FIG. 2b and encoded in LUT 206, which stretches thesaturation vector in a hue direction compared to some white point. Weassume for simplicity we have defined our uv space in a cylindrical onewith the maximum saturation on the periphery (and coded as 1.0). Ofcourse the skilled person will understand that we can both encode oursaturation strategy in another colorimetric definition, or given thatthe definition is e.g. in cylindrical Y′uv space, the designer of thedecoder hardware or software can choose to actually perform itequivalently in another color space, such as the RGB-based YCrCb space,etc. The grader can also determine and encode in S_im luma-dependentsaturation strategies, i.e. functions changing the saturation, whichmultiplicator varies with the luminance of the processed color.Basically, a more advanced embodiment of S_im will have a saturationencoding structure. This may be e.g. a web-based definition which hasfor a number of key hues (e.g. the 6: RGBCYM) a multiplier functiondefined over luma: MS(Y′). From this which can be encoded as 6 LUTs ofvalues similar to 206, at the receiving end the decoder can determine asaturation strategy for all colors in the gamut by interpolation. A morecomplex strategy may even introduce variability of the saturation in theradial direction. This can be easily encoder by determining thesefunctions (similar to what one sees in FIG. 2b , but now variable overthe luma height in the gamut) simply parametrically, e.g. as offset,gamma, gain functions. In this case one would have: s_out=s_in+F(s_in,Y′) for the key hues, and in case of e.g. a three-parameter functionshape control, one may encode this in S_im either as 3×6 LUTs specifyingthe luma behavior of e.g. the saturation gamma parameter as varying overY′, or 6 LUTs for the hues, but where not a single multiplicative valueis encoded at each position, but a triplet [sat_offset(Y′_i),sat_gain(Y′_i), sat_gamma(Y′_i)] LUT of yellow, consecutively over anumber of positions i sampling the possible lumas in the gamut.

Now in some embodiments of an encoder (and corresponding decoder) thereis an optional transformation to u′v′ for the color characteristics ofthe pixels, which we will now elucidate (but other embodiments mayalternatively or additionally encode in e.g. R′G′B′ or YCrCb, etc.directly, and not even have the optional unit 107 inside; note also thatsome Yu′v′ processing can be mathematically re-written as equivalentlinear RGB processing).

Having applied to dynamic range transformation to create the right LDRlook (e.g. in RGB space, or XYZ etc.), assuming we didn't already do themapping in Y′uv space, color transformation unit 107 of the exemplaryelucidation embodiment will do the conversion to our u′v′representation, with the lumas Y′ in that color representation beingdetermined by our total tone mapping function (i.e. the lumas ofintermediate LDR image LDR_i), and u,v as per the above equations. Wecould also do colorimetric transformations in unit 107, which conditionthe colors already when a different device dependent RGB or multiprimaryspace is envisioned. E.g. if our M_HDR was encoded with a smaller RGBtriangle, but the LDR is for a wide gamut display, the grader mayalready predefine a saturation boosting strategy, although things willoften be the other way around, in which case unit 107 may implement achromatic gamut mapping.

Finally the resulting LDR_uv is encoded with a classical LDR image orvideo compressor 108, i.e. typically DCT or wavelet transformed etc.

This compressed image LDR_c is send to a formatter 116, which adds themetadata on the applied mapping function according to a standardizedformat, for it to be suitably available at a receiving side. I.e. thisformatter adds the sensitivity value (RHO or alternatively SENS), thefurther tone mapping for fine tuning the LDR look as determinedtypically by the human grader (although in the further future someencoders may be smart enough to do some fine-tuning themselves) withfunction defining parameters 205 typically as a LUT of values (F1, F2, .. . ), the saturation encoding 206, e.g. also a set of parametersdefining a multi-linear function, etc.

The further tone mapping for technical reasons is typically storedseparately in the image or video signal S_im, preferably as a set ofinteger or real values 207, which may be used to store e.g. a 256-pointor 1024 point LUT.

The coded LDR_c can be decoded again to LDR_d, and then upgraded bycolor mapping unit 118 so that the grader can see via image output 119what the reconstructed HDR Rec_HDR would look like at a receiving end.If he so desires he could even test the influence of some typicalcompression settings up to e.g. strong compression. The herein describeddecoder could also be used in a re-coding strategy, where the gradinglook may already have been prepared previously, but now e.g. a lowquality highly compressed LDR version is redetermined for someparticular image/video communication application. That secondary gradermay even re-tune the parameters. Depending on whether he has theoriginal M_HDR available he may e.g. redetermine the downgradingfunctions to achieve a new more appropriately adjusted LDR look (e.g.serving mobile phone viewers), and in fact he may even do that when onlyhaving the good Rec_HDR available instead of M_HDR. The split of atechnical grading part to more appropriately allocate the luma codes isvery useful for such scenarios. Because the functions mapping to LDR_o(and the corresponding close reconstruction LDR_ul thereof) determinethe actual artistic LDR look, and they may have been determined once andfor all by the primary grader at or around the time of initialproduction of the content. But the encoder can still automatically orsemi-automatically with involvement of the secondary grader determinethe technical mapping with the small modifications like 301, and thecorresponding LDR_i (or LDR_t), and the coded metadata Ff1, Ff2, in setof real or integer values 207 in S_im, which may of course be differentfor different technological limitations, such as the amount of bits(e.g. only 8 bits for the luma channel).

The decoder 150 may be an IC in, e.g. such as in this elucidation, asettopbox or computer connectable to a display 160 or television (sowhen we say decoder we intend to cover both any small realization ofthis such as a “settopbox on a USB stick” or any large apparatusrealizing and benefiting from our invention such as a settopbox withhard disk and optical disk reading facilities, and encoder can beanything from a small device to a large grading system, etc.), but ofcourse the television may not be a dumb monitor but comprise all thisdecoding technology in its own IC. The display 160 may be both an LDRdisplay or a HDR display, or basically any display connected via anyimage communication technology via image output 157, such as e.g.wireless streaming to a portable multimedia device or a professionalcinema projector.

The decoder gets our formatted S_im via image input 158, and adeformatter 151 will then split it in an image LDR_c (IMG in FIG. 2) fordecompressing by a classical JPEG-like or MPEG-like decompressor 152,and the parameters P from metadata (e.g. a sensitivity setting 1000, andsome values which can be used to reconstruct a tone mapping orsaturation mapping functional shape). Optionally in the decoder is thetone remapping unit 159, because since this technical remapping isusually not a severe deformation of the grader-intended LDR look LDR_ul,some decoders can afford to ignore it. Fully HDR compliant decodersshould however use this unit 159 to apply a technical re-correctionstrategy as codified in the Ff values of 207, to arrive at the correctLDR look LDR_ul (which is a close approximation of LDR_o). Thiscorrected LDR image (LDR_ul) goes to a further display color tuning unit154. This unit 154 can apply the needed optimization for a particularsay 1300 nit wide gamut display (tunability). Although variants arepossible, we have drawn a typical decoder for our HDR encodingphilosophy, which has an image processing path for recovering LDR_ul (orif 159 is not present its approximation LDR_t), but also has a secondimage processing path to determine the Rec_HDR. This is done in dynamicrange conversion unit 153, which typically applies the inverse mappingsapplied at the encoder (actually in the signal one will typically encodethe parameters of this inverse mapping, i.e. an upgrading). The displaycolor tuning unit 154 will typically be arranged to combine theinformation in the two gradings, which could be done based on using onlyone image and the color mapping parameters P, but we assume in thiselucidated embodiment that it gets a Rec_HDR and LDR_ul image as inputand then interpolates those, according to which display with which peakbrightness is connected and to be supplied with the appropriately gradedimages.

Apart from tone mapping to obtain the correct brightness look, a colortransformation unit 155 may typically be comprised arranged to dochromatic adaptations to optimize for a different color gamut than theencoding gamut (e.g. Rec. 2020 to DCI-P3 or Rec. 709, etc.).

What will be output via image output 157, and hence calculated by unit154 will of course depend on the connected display. If it is an LDRdisplay, unit 154 may send e.g. LDR_ul, after of course correct colorremapping (by unit 155) from Y′uv to a particular device dependentR′G′B′ encoding e.g. If the display 160 connected is close to a 5000 nitpeak brightness display (see also on how the decoding apparatus can aska t.v. its capabilities in WO 2013/046096; a controller 161 can do suchcommunication with the display and even with the viewer to obtain hispreferences, and may be arranged configure how the display tuning unit154 should behave and which kind of image look it should calculate andoutput) the Rec_HDR look image may be output, again after suitableformatting according to what the television wants to receive (i.e. thiscan still be an Y′uv encoding, e.g. our S_im format with now an HDR lookimage stored in 201/IMG, and some functional metadata may also betransmitted so that the television can do some last look colorimetricfine tuning based on the information on how gradings change over thespectrum of rendering possibilities as encoded in this metadata, or itcan already be an R′G′B′ HDR display driving image). For intermediatepeak brightness displays, unit 154 may output a suitable driving image,again either in our Y′uv format, or another format.

Finally, the content creator may prescribe in the signal whether hedesires that the compensation mapping of unit 159 should not be skipped,e.g. because the content creator thinks that LDR_t seriously deviatesfrom LDR_ul. This can be done by encoding a Boolean 209 in anIGNORE_TECHNICAL_MAPPING field of the metadata.

It should be clear to the reader that where we have elucidated only theminimum of one set of parameters, of course along the same rationaleseveral sets of color mapping functional metadata can be encoded inS_im, e.g. one set for going from the sole image IMG (being an LDRimage) to a reference e.g. [0-5000] nit HDR look image, and a second setcan be added for going to e.g. a 1500 nit MDR look. And although doing aspecific decomposition of a sensitivity, gamma, gain, and further finetuning function shape is advantageous, and at least good for technicalelucidation, any one of the mappings, e.g. the mapping LDR-2-MDR mightbe encoded in S_im in a condensed form, e.g. by only filling the tonemapping LUT or set of values 205, which codify the final mappingfunction (i.e. everything of sensitivity, fine tuning, and technicalmapping together).

FIG. 4 schematically shows a typical embodiment of our decoder core unit400 (in this example the minimal part of mode ii, without technicalregrading, or Yu′v′ conversion, etc.). After a decompressor 401 doingthe run length or arithmetic decoding, and inverse DCT etc., we get animage LDR_t, which we will assume to be in gamma 2.2. representation(i.e. with lumas or R′G′B′ components defined according to Rec. 709) andnormalized. There may be a first control unit 420, which can directlysend this image to a connected LDR TV 410 (directly meaning there may ofcourse be some legacy formatting involved; in principle LDR_t could alsobe e.g. a linear image, in which case their will be a need tore-gamma-2.2 map it before sending it to the LDR display, but it may beadvantageous if that is not needed; the further tone mapping functionswill typically be different depending on what type LDR_t is, which mayalso be indicated with an indicator IND_2 in S_im). Then a first tonemapping unit 402 does the inverse mapping of the arbitrary tone mapping,the defining parameters of that function shape P_CC being received inthe metadata MET(F). Then a second tone mapping unit 403 does a tonemapping re-darkening the darker colors relatively to the brighter ones,by e.g. applying the rho-equation above, with a received RHO value. Theunit could also calculate the RHO value from a received display peakbrightness PB_HDR, received from the connected HDR display 411. Then athird tone mapping unit 404 performs a gamma power function with areceived GAM value being e.g. 2.4 preferably. Then a multiplier 405 maydo a multiplication with GAI, which by default may be 1.0. Optionally acolor saturation processor 406 may do some saturation processing.Finally control unit 421 may send the image to the HDR display 411, andit may do some further processing, e.g. to correctly format the imageaccording to a standard which the display understands, e.g. over a HDMIconnection.

FIG. 6 shows a simple encoder dynamic range conversion unit embodiment.It comprises a normalization unit 601 for normalizing all colorcomponents to 1 (i.e. if e.g. R, G, and B are normalized to 1.0, so willthe maximum luminance be normalized to 1.0, and vice versa). Thenormalized luminances Yn_HDR of HDR image pixels (or in equivalentembodiments e.g. the normalized linear RGB components) go to a firsttone mapper 602 doing a gamma operation, with a gamma as desired by thegrader (or automatic grading unit), but usually fixed to 1/(2.4). Then asecond tone mapper 603 does the transformation which appropriatelybrightens the HDR dark colors, e.g. with

${v = \frac{\log( {1 + {( {{RHO} - 1} )*{xg}}} )}{\log({RHO})}},$with an appropriate RHO factor proposed by the grading system dependingon the dynamic range difference between (the peak brightness of) M_HDRand the typically 100 nit LDR, and typically ultimately accepted by thegrader, who may or may not change this initially proposed RHO value.Then by using third tone mapper 604 the grader starts fine-tuninglooking at various objects in the image, and ultimately defines a customtone mapping curve CC, by changing various lumas of those variousaccording to the grader important image objects. This yields the lumasYn_LDR_of the LDR_o image, with all data ready to be encoded.

The algorithmic components disclosed in this text may (entirely or inpart) be realized in practice as hardware (e.g. parts of an applicationspecific IC) or as software running on a special digital signalprocessor, or a generic processor, etc.

It should be understandable to the skilled person from our presentationwhich components may be optional improvements and can be realized incombination with other components, and how (optional) steps of methodscorrespond to respective means of apparatuses, and vice versa. The word“apparatus” in this application is used in its broadest sense, namely agroup of means allowing the realization of a particular objective, andcan hence e.g. be (a small part of) an IC, or a dedicated appliance(such as an appliance with a display), or part of a networked system,etc. “Arrangement” is also intended to be used in the broadest sense, soit may comprise inter alia a single apparatus, a part of an apparatus, acollection of (parts of) cooperating apparatuses, etc.

A computer program product version of the present embodiments asdenotation should be understood to encompass any physical realization ofa collection of commands enabling a generic or special purposeprocessor, after a series of loading steps (which may includeintermediate conversion steps, such as translation to an intermediatelanguage, and a final processor language) to enter the commands into theprocessor, and to execute any of the characteristic functions of aninvention. In particular, the computer program product may be realizedas data on a carrier such as e.g. a disk or tape, data present in amemory, data traveling via a network connection—wired or wireless—, orprogram code on paper. Apart from program code, characteristic datarequired for the program may also be embodied as a computer programproduct. It should be clear that with computer we mean any devicecapable of doing the data computations, i.e. it may also be e.g. amobile phone. Also apparatus claims may cover computer-implementedversions of the embodiments.

Some of the steps required for the operation of the method may bealready present in the functionality of the processor instead ofdescribed in the computer program product, such as data input and outputsteps.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention. Where the skilled person can easilyrealize a mapping of the presented examples to other regions of theclaims, we have for conciseness not mentioned all these optionsin-depth. Apart from combinations of elements of the invention ascombined in the claims, other combinations of the elements are possible.Any combination of elements can be realized in a single dedicatedelement.

Any reference sign between parentheses in the claim is not intended forlimiting the claim. The word “comprising” does not exclude the presenceof elements or aspects not listed in a claim. The word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements.

The invention claimed is:
 1. A method of encoding a high dynamic rangeimage (M_HDR), comprising the steps of: converting the high dynamicrange image to an image of lower luminance dynamic range (LDR_o) byapplying: a) normalization of the high dynamic range image to a scale ofthe luma axis being [0,1] yielding a normalized high dynamic range imagewith normalized colors having normalized luminances (Yn_HDR), b)calculating a gamma function on the normalized luminances yieldinggamma-converted luminances (xg), c) applying a first tone mappingyielding lumas (v) which is defined as v=log(1+(RHO−1)*xg)/log(RHO),with RHO having a predetermined value, and d) applying an arbitrarymonotonically increasing tone mapping function mapping the lumas tooutput lumas (Yn_LDR) of the lower dynamic range image (LDR_o); andoutputting in an image signal (S_im) a codification of the pixel colorsof the lower luminance dynamic range image (LDR_o), and outputting inthe image signal (S_im) values encoding the function shapes of the abovecolor conversions b to d as metadata, or values for their inversefunctions, which metadata allows a receiver to reconstruct areconstructed high dynamic range image (Rec_HDR) from the lowerluminance dynamic range image (LDR_o), wherein RHO or a value being afunction of RHO is outputted in the metadata.
 2. A method of encoding ahigh dynamic range image (M_HDR) as claimed in claim 1 in which thegamma function calculation uses a gamma value equal to 1/(2.4).
 3. Amethod of encoding a high dynamic range image (M_HDR) as claimed inclaim 1 comprising determining a gain value (gai) for mapping themaximum luma of the lower dynamic range image (LDR_o) to a specificvalue of the possible values in the reconstructed high dynamic rangeimage (Rec_HDR), and encoding that gain value in the image signal(S_im).
 4. A method of encoding a high dynamic range image (M_HDR) asclaimed in claim 3, in which the identification of the first geometricalregion is performed by a human grader via a user interface unit, and theamount of banding of the first geometrical region in the reconstructedhigh dynamic range image (Rec_HDR), and the visual quality ofreconstruction of the second geometrical region in the reconstructedhigh dynamic range image (Rec_HDR) are judged by the human grader asacceptable or unacceptable, wherein in case of the acceptable judgementthe values encoding the function shape of the redistribution mappingfunction or its inverse are encoded in the image signal, or in case ofthe inacceptable judgement the steps are done again with differentparameters to come to an alternative redistribution mapping function. 5.An image decoder as claimed in claim 4 comprising a tone remapping unitarranged to apply a further tone mapping (Ff1, Ff2, . . . ) received inthe image signal (S_im) to the lower dynamic range image (LDR_t) toobtain a second lower dynamic range image (LDR_ul) which reverses a coderedistribution action previously applied by the encoder of the lowerdynamic range image for obtaining a reduced banding in at least a regionof the reconstructed high dynamic range image (Rec_HDR).
 6. The methodof encoding a high dynamic range image (M_HDR) of claim 1 comprising:after applying any of the above color mappings to determine the lowerdynamic range image (LDR_o), applying a further technical tone mapping(301) to determine a second lower dynamic range image (LDR_i) which canbe used to drive LDR displays as an alternative driving imagealternative to the lower luminance dynamic range image (LDR_o), whichtechnical tone mapping is determined by: a) determining a firstgeometrical region of the lower luminance dynamic range image (LDR_o)for which the visibility of banding in the corresponding reconstructedhigh dynamic range image (Rec_HDR) is above an acceptable level, b)determine a range of lumas (L_u) for that region, c) determine a secondrange of pixel lumas (L_uu) adjacent on the luma axis to the range oflumas (L_u), wherein the second range is identified to fulfill theconditions that it has a number of lumas above a minimum number (MIN),and corresponds to a second geometrical image region which contains atexture which can be represented using less than the minimum number ofcodes in an LDR image (LDR_i) upon which to apply the functions yieldinga reconstructed high dynamic range image (Rec_HDR) of sufficient visualquality for that second region, and d) determining a redistributionmapping function which redistributes the lumas of the first and secondrange of lumas, so that additional codes are available for the firstrange, and outputting in the image signal (S_im) values encoding thefunction shape of the redistribution mapping function or preferably itsinverse.
 7. An image encoder arranged to encode a high dynamic rangeimage (M_HDR), comprising: a dynamic range conversion unit arranged toconvert the high dynamic range image to an image of lower luminancedynamic range (LDR_o), the dynamic range conversion unit comprising: a)a normalizer arranged to normalize the high dynamic range image to aluma axis ranging over [0,1] and to output normalized luminances(Yn_HDR), b) a gamma conversion unit arranged to apply a gamma functionto the normalized luminances and to output gamma-converted luminances(xg), c) a first tone mapping unit arranged to apply a first tonemapping which yields lumas (v) which is defined asv=log(1+(RHO−1)*xg)/log(RHO), with RHO having a predetermined value, d)an arbitrary tone mapping unit arranged to apply an arbitrarymonotonically increasing function which maps the lumas (v) to outputlumas (Yn_LDR) of the lower dynamic range image (LDR_o); and the imageencoder further comprising: an image compressor arranged to apply a datareduction transformation to the colors of the lower dynamic range image(LDR_o) which colors are organized in component images, and whichreduction transformation involves at least applying a DCT transform toblocks of adjacent color component values, yielding a compressedcodification (LDR₁₃ c) of the pixel colors of the lower luminancedynamic range mage; and a formatter arranged to output in an imagesignal (S_im) the compressed codification (LDR_c), and arranged to inaddition output in the image signal (S_im) values encoding the functionshape of the color conversions as metadata, or values for their inversefunctions, which metadata allows a receiver to reconstruct a highdynamic range image (Rec_HDR) based upon the lower luminance dynamicrange image (LDR_o), the values comprising RHO or a value being afunction of RHO.
 8. An image decoder arranged to receive a high dynamicrange image signal (S_im) and comprising: a deformatter arranged toobtain a compressed pixellized lower dynamic range image (LDR_c) andparameter data (P) out of the image signal (S_im); and a decompressorarranged to apply at least an inverse DCT transform to the compressedpixellized lower dynamic range image (LDR_c) to obtain a pixellizedlower dynamic range image (LDR_t); and a dynamic range conversion unit(153) arranged to transform the lower dynamic range image (LDR_t) into areconstructed high dynamic range image (Rec_HDR), wherein the dynamicrange conversion unit comprises: a) an arbitrary tone mapping unitarranged to apply an arbitrary monotonically increasing tone mapping,the parameters which define it (P_CC) being received in the parameterdata (P), b) a first tone mapping unit arranged to apply a mapping asdefined by a function of the form:${{xg} = \frac{( {{{power}( {{RHO},v} )} - 1} )}{( {{RHO} - 1} )}},$win RHO being constant received in the parameter data (P) and c) a gammaconversion unit arranged to apply a gamma mapping with a received gammavalue (GAM).
 9. A method of decoding a high dynamic range image signal(S_im) comprising: obtaining a compressed pixellized lower dynamic rangeimage (LDR_c) and parameter data (P) out of the image signal (S_im);decompressing the compressed pixellized lower dynamic range image(LDR_c) by applying at least an inverse DCT transform to the compressedpixellized lower dynamic range image (LDR_c) to obtain a pixellizedlower dynamic range image (LDR_t); and transforming the lower dynamicrange image (LDR_t) into a reconstructed high dynamic range image(Rec_HDR), by: a) applying an arbitrary monotonically increasing tonemapping, the parameters which define it (P_CC) being received in theparameter data (P), b)applying a mapping as defined by at least onereceived parameter (RHO) the mapping being defined by a function of theform:${{xg} = \frac{( {{{power}( {{RHO},v} )} - 1} )}{( {{RHO} - 1} )}},$and c) applying a gamma mapping with a received gamma value (GAM), whichis preferably equal to 2.4.